Large Scale Central

Steam Locomotive Anatomy 101

I am going to endeavor on writing up a little tutorial about the steam locomotive anatomy. I have always loved steam engines from a time before I can remember. Not just locomotives, but just about anything powered by a steam boiler such as a tractor or even a saw mill. When I got into modeling I knew I was going to model steam. And at first I was just going to glue on the parts in the places that seemed reasonable. But I wanted more than that. In my modeling I wanted to learn. Thanks to google searches and a tremendous amount of help and knowledge from this group, and the MyLargeScale Master Classes David Fletcher produced; I learned a lot about locomotive anatomy and what all the bits and pieces do.

But like me, many people are either new to trains, new to steam, or just have never been taught what the gizmos are. Jim Rowson inspired this tutorial when he asked about Cliff’s backhead and appreciated Rick Marty and I “discussing” what various gizmos did when we built our steam cranes. So armed with this idea I decided to do Jim a favor and anyone else who wants a very very basic understanding of steam locomotive anatomy and operation.

Disclaimer: I don’t know everything, I don’t even know a lot, I can be wrong on some of the specifics. I know enough to have a basic modelers understanding. There are folks on here who have a much better handle on this subject than I do and I hope they chime in and add to or correct my information. I am still a student.

So welcome to Steam Locomotive Anatomy 101

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I drew up this picture to give us a start. It is a very basic boiler that will be found in the simplest of locomotives. Its not to scale, its not anatomically correct, but it shows and highlights in simple terms the major pieces we all talk about when were refer to our beloved Steam locomotives.

The boiler as I refer to it here is the combination of the firebox, water vessel (help here with the proper name), and the smoke box. Included here are some of the appurtenances that are attached.

So the main long cylinder I call the “water vessel” Is the heart of it. It is where the water and steam are contained. It is filled with water to a point were it is not full but mostly full with water. They leave the top portion of this chamber empty of water to give a place for the water to flash to steam. If it were full the water would heat but have no room to expand to steam and stay liquid. When water flashes to steam it creates a significant amount of pressure due to the increased volume of the steam. The same amount of water (number of H2O molecules) is in the boiler when it is cold or when it is hot, the water has the same mass. As the water flashes to steam the molecules actually become significantly larger, this increases the volume (not the mass) and creates pressure inside the boiler. That pressure is then used to do the work of driving cylinders and other devices on the locomotive.

The boiler vessel is not just a hollow water tank. It has a series of tubes called fire tubes that run the length of it. These tubes collectively are the “heat exchanger”. Hot air/gas/smoke pass through them and in turn heat up the water to the flash point. If you remove the backhead of the locomotive or look in the smoke box door you see these tubes.

Attached to the back of boiler tube is the “firebox”. As its name implies this is where the fire is made and tended. Its obvious location is in the cab where the people are working so they can stoke the fire. Many different things are used to create the fire. I am pretty sure that if it burns it has been used to power a locomotive. Common fuels are coal, wood, and oil. In the fire box you light a fire and that’s pretty much all it does, its the stove. The fire tubes attach to the back wall of the fire box and the smoke generated from the fire passes out of the firebox and into the tubes through the boiler tube to the “smoke box”.

The smoke box is attached to the front of the boiler tube. It collects the smoke coming through the fire tubes and allows it to move up the chimney (or smoke stack if you prefer). It does a few other things but we will get to that more when we get to the actual operation. For now we will say it is a place to collect smoke (and exhausted steam) for elimination through the chimney.

One of the other things we see is the “steam dome”. If the boiler tube is the heart of a steam locomotive then the steam dome is its lungs. Steam is lighter than liquid water so it sits on top of the water in the boiler. The steam dome is a collection point for that rising steam to gather in quantity so it can be used for driving the locomotive. There is a pipe with a valve inside there where steam can rise into the dome then by pressure and force enters that pipe. The valve is attached to the throttle lever inside the cab. Open the valve and more steam is allowed in the pipe, close the valve and less steam can enter. This regulates the amount of steam that can be delivered to the cylinders and it increase the speed (or decreases it). On top of the steam dome you will find at least one, usually two “pop off valves” (again help here with the proper name). These valves are set at predetermined pressure settings and as the boiler reaches those pressures they open, bleeding off the pressure. When there is more than one they are set to incrementally greater pressures. The purpose of these valves is to keep the boiler from over pressurizing and exploding. Another thing commonly found on the steam dome is the steam whistle.

Another thing you see on the boiler is the sand dome. It holds sand, that sand is fed through tubes to the drivers to give them traction. Metal on metal is slippery, the sand adds grit to give the metal drivers traction and keeps them from slipping until they can overcome the weight of the locomotive and inertia takes over and they no longer slip. In most cases the sand is gravity fed to the drivers. Sometimes they can be pressurized with air (not steam or the sand will get wet and clump) and the sand squirts out the tubes.

The chimney, or smoke stack, is just like that of a stove; its where the exhausted smoke is let out.

Usually a boiler is insulated to help make it more efficient. This insulation is then wrapped in a thin piece of steel. This is the boiler jacket. It s why the smoke box has a smaller diameter than the rest of the boiler. The boiler bands are there to keep the jacket on.

This is all I have time for at the moment, but it will give us the starting point for further discussion when we get into all the pipes and stuff that is attached to the boiler. We will start that discussion with all the stuff we see attached to the backhead.


For the next installment we are going to talk “Back Heads”. I am not sure if this term is an official term or not but it is the common term used for the back of the boiler/fire box. For the purposes of this discussion I will not only be referring to the very back/wall surface, but will include the top area of the rear part of the boiler that is inside the cab. Many things are attached here and it is easier to collectively call this entire area the “back head”.

Here is a good picture of a simple back head arraignment. First, we must give credit where credit is due. This picture was taken from Flicker and the photographer is Joe Ross.

I have added some labels to Mr Ross’ picture. The link takes you to the original as well as some other fine pictures of this locomotive.

I am not going to make any attempt at all to tell you what every knob, lever, gauge, pipe, and etc are. One its to deep for what we are trying to accomplish and the most important reason is . . . Well because I don’t know. But here are the basics.

“Gauge Manifold” this is nothing more than a cluster of steam and air pressure gauges. They are the dashboard and are relaying information to the engineer and fireman about whats going on with their iron beast. For this discussion the only one I care about is the big one that is hidden. This is the boiler pressure gauge. It does what the name implies, it tells the crew how much pressure is inside the boiler. It tells them how much power they can generate, tells them if they need to stoke the fire and get some more pressure, tells them if they need to shut the dampers and lower temp to prevent an explosion. If we called the boiler the heart of the locomotive then this is the EKG.

In front of the gauge manifold is a brass gizmo that is a cylinder with a brass light bulb looking thing on top. This is a “hydrostatic oiler”. It is filled with oil, where it is thinned by the heat of the boiler, and an inlet pipe adds steam pressure. There is an outlet pipe that delivers the oil to points beyond via that steam pressure.

Behind the hydrostatic oiler attached to the top of the backhead/boiler is the “turret”. Turrets come in all shapes and sizes, from fancy cast brass devices to simple pipe fittings. What they do is to take steam off the top of the boiler and run it to a bunch of valves that then distribute it to all the various components that are run off steam. Some of these things are: the hydrostatic oiler, the brake air pump, the dynamo/generator, etc.

On the left behind the guy is the feed water “injector”. I will come back to it later, it deserves its own discussion with pictures.

On this locomotive there are two main “sight glasses”. What these do is rather simple. It is a glass tube with a valve at either end and a drain at the bottom. Water seeks its own level. So the water in the boiler is let in the lower tube and steam in the top, which correlates exactly to the same levels in the boiler. It is a visual representation of the level of water in the boiler. It lets the crew know when to add water.

A redundant system to the sight glass is the “tri-cocks and funnel”. This is an arraignment of three valves that correspond to the same levels as the sight glass: one at the full point, mid point, and top of the crown sheet. We will take about boiler level land what the crown sheet is and why it is important in a different discussion. For now we will call the lower valve as empty. The way this works is each of these valves is cracked open just enough to get a steady drip of water. When all three are dripping the boiler is full. When the top one is hissing steam and the two lower ones are dripping you are at the optimal level for safe operation and performance. When the top two are hissing steam and the lower one dripping its time to add water to the boiler. If all three are hissing steam you are in trouble and a boiler explosion is possibly forth coming due to lack of water over the crown sheet (again we will discuss this later). The funnel just catches the dripping water and drains it out through the floor of the cab.

At the lower center of the back head is the fire box door. Its a simple as the door to a stove. You open it to add fuel and it often times has an incorporated series of holes to act as a damper to let in or restrict air flow depending on what you need the boiler to do. There are a whole host of differe3nt types of doors. Simple ones with a latch and hinges to a clam shell affair that you use your foot to actuate a lever to open the doors.

Finally we see the brake. This discussion, like the injector, will come later.


Well done, Devon! :+1:

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Excellent article Devon.

Next up will will start talking about some of the major components of the plumbing system as we leave the back head. The first of those is the “injector”. Like everything else there are different styles and manufacturers but they all do the same thing in basically the same way. But first some background. The purpose of the injector is to “inject” water from the tender into the boiler. The injector is the result of an evolutionary process to overcome a simple problem. The simple problem is how to get water at a lower pressure into a vessel at a higher pressure. The water in the tender is open to atmosphere and has no pressure. The boiler vessel has a pressure ranging anywhere from 150-160 psi to high pressure boilers over 350 psi. Simple physics will tell you which way water will want to flow, it flows from high to low pressure. So the natural state of a locomotive and its tender is for the locomotive to want to push water into the tender, not the other way around. So how do we do it?

In the very earliest days of steam power, you stopped your machine, depressurized it, opened the lid, filled it with water, closed it and pressurized. Obviously this did not last long. Hand operated mechanical pumps began to be used to generate the higher pressure needed to overcome the boiler pressure. Some of the earliest ones were a simple hand operated lever pump. You just start cranking the handle and pumping it in. Again this didn’t last long. A mechanical device was needed and a neat device called the cross-head pump was invented.

Piciture credit:

This device used the motion of the crosshead connecting rod to drive a piston pump that would suck water from the tender and inject it into the boiler up front near the smoke box. It worked, but it was a mechanical device with moving parts and was remotely located away from the crew. The next evolution was the “injector”. Here is just one example of one type, a Nathan, there are many.

Photo from Discover Live Steam

They work by use of a venturi. Steam is passed by a small orifice that the tender feedwater line is attached to. I won’t go into detail on how a venturi works other than to say it causes a negative pressure on the tender feedwater line and sucks the water from the tender. As it exits the venturi, it is now at a greater pressure than the original steam inlet was and it can deliver the water it is sucking into the boiler.

Photo from wikipedia.

From the diagram we see that there is a feed water inlet pipe from the tender (or saddle tanks). There is a steam inlet pipe from either a valve on the side of the boiler or the turret. There is the outlet pipe that leads into the boiler usually at the front of the boiler just behind the smoke box. And there is an overflow pipe. When the crewman pulls the lever it opens the inlet steam valve and allows steam to start passing across the venturi which pulls water from the tender. As a note here, where the water enters the boiler there is a check valve to prevent the process from trying to reverse.

check valve
Picture from York Blog taken by S.H. Smith.

The injector is operated with a lever that opens and closes the steam inlet. It was most common as development of locomotives progressed to have two injectors. One on each side so that either the engineer or the fireman could operate it. Sometimes they were in the cab right on the side of the boiler. Or they were outside of the cab to the front on the side of the boiler and had a longer handle that extended into the cab.


Jeez, Devon, now you’re making the rest of us look like pikers! :smiley:

This is a VERY nicely done series.

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Now time for some practical application to put the parts we have so far into a working order. A steam locomotive works off steam being at high pressure. High pressure moves things. We put water in a boiler. We light a fire in the firebox. That creates hot gas (smoke) which is drawn by draft through the fire tubes up to the smoke box and out the chimney. In doing so those fire tubes heat the water in the boiler to temps above boiling and steam is produced. That steam is then piped to various things to “move” them in such a way that we accomplish the needed task. In the case of the locomotive cylinders (the primary reason for the steam) it pushes a piston back and forth as it fills a chamber and then exhausts that chamber. We will go into more detail about steam chests and cylinders later. As this work is accomplished water/steam is consumed and exhausted. A boiler needs to have the right amount of water in it to operate efficiently. We have already discussed this. As water is consumed it must be replaced. So as the train crew monitors the sight glass and tri-cocks, they are aware that it is time to fill the boiler. When that time comes, they grab the lever on the injector give it a pull and fill the boiler until it is back where they want it.

I said I would come back to the Crown Sheet and why it must be kept in water. The Crown Sheet is simply the top (crown) sheet of metal that makes up the fire box. It must remain submerged in water so that it can not super heat and unexpectedly over pressurize the boiler. So the purpose of everything we have mentioned so far is to prevent this from happening. Another note here is on small mountain railroads, locomotives encountered steep grades. One issue with this was if the locomotive was pointed nose down the grade the water would slosh to the front and expose the crown sheet and boom. So it was not at all uncommon on steep mountain railroads to see locomotives backing down the grade to keep their nose pointed up hill and keep the water sloshed to the back and over the crown sheet.

So now we have pretty well covered the parts and pieces that keep the boiler full of water and producing steam. Next up will start talking about the main reason to have steam, driving the cylinders which make the locomotive go.


Fantastic trove of info Devon! Thanks! I’m looking forward to the grand climax where we find out that the engineer did it in the tender with a Johnson bar!

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I’m doing it for you Jim. Thank you for the complement. I know when I started asking these questions people stepped up and explained it to me. One thing I hate is when people say “Google it”. First off whats the purpose of a discussion forum if we won’t discuss it. Second, how does it end up on a Google search, if someone doesn’t write it down and put it on the web to be searched. So I offer this because you are my friend and you asked for the information. And I would never tell my friend to google something, but rather I would discuss it with them. So this is me discussing it with you. And for anyone else, who like me, wanted to know but didn’t want to just google it.


No wonder steam was replaced. Although there was a study in the 1970’s by the BN to make a nuclear powered steam engine/turbine. Never made it past the conceptual stage but there’s a report about it floating around the internet.

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PERFECT attitude - I REALLY like this. You are right - too many folks tell you to look it up. I remember folks telling me that when I asked how to spell a word! Hey, if YOU don’t know, please don’t tell me to ‘look it up’.

THIS is what a forum is for - discussion!

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Yeah diesels are easy, get in turn the ignition key put her in drive and step on the gas pedal.

I want to throw out this resource here. I mentioned David Fletcher (Australian G gauge modeler) who did a series of “Master Classes”. One of them in particular is his 2-6-0 Mogul build. Chapter six of that class, found here is a great resource to locomotive plumbing and how to model it.

I am relying on his description in that class for this next part. A final piece of plumbing coming off the turret is what he refers to as the blower pipe. What the blower pipe does is act like a super charger. Some back ground; we talked about the fire being pulled through the boiler by draft created when the fire is made in the fire box and smoke exhausted out the stack. A way of super charging this draft is to exhaust the steam from the steam chests/cylinders into the smoke box. This is one of the “other” things I said the smoke box does. Steam enters the bottom of the smoke box through the saddle. I will talk about this more when I talk about the cylinders. But for now that steam enters the bottom and rushes out the top of the chimney. It is white the “smoke” from a locomotive often times is more white/grey than black. Remember our friend the venturi? Well that steam rushing by the fire tubes creates a venturi and increases the draft by sucking the fire through the tubes, in effect we can think of this as a turbo charger. But how do we create this venturi effect when the throttle is off and no steam is going to the cylinders? We still want to rev up the engine but we don’t want to go anywhere. We want to do this so we can build up steam prior to taking off? Enter the “blower valve and piping”.

A valve is fixed either to the turret or the side of the boiler with a pipe that runs to the smoke box. and into the bottom of the chimney. At idle the valve is opened and steam is allowed to rush down this pipe and out the chimney. As it rushes out the chimney we again get a venturi that pulls the hot gas through the fire tubes and out the chimney. It increases the draft of the locomotive and in turn raises the temp in the boiler which in turn increases the pressure. So at idle the locomotive can be “given a little gas” and be “reved up” prior to actually opening the throttle and driving the locomotive. Prior to the blower valve a locomotive had to get under way to start the turbo charger effect. This valve and piping most typically was on the fireman’s side and ran under the walks and it is a reason its hard to find a picture of. That and not all locomotives employed a blower system.

That should do it for the basic stuff coming off the back head.

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Nice work Devon.

I do recall there is a very good offline resource in the Model Railroader Cyclopedia - the first few sections describe all the plumbing and controls on a typical locomotive. That book apparently was started in 1936 and continuously updated. I think I got mine in the 1980s.

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Now we can get going on the cylinders and steam chests. If the boiler is the heart and steam dome the lungs, we could say the fire box is the belly, and the plumbing is the arteries and veins. To keep up the analogy, the cylinders and the steam chests are the muscle. Included in this discussion we will include the boiler/cylinder saddle. First a couple of pictures of the pieces.

Picture for the San Diego Railroad Museum
The saddle does a few things. One of its most important jobs is a support for the boiler. It is a heavy cast iron affair that is bolted to the locomotive frame and supports the weight of the boiler at the front. It also acts as a hanger for the cylinders and steam chests. But it is not a solid bit of iron. It is rather a set of “pipes”. In the line drawing you can see that one set of pipes connects to the steam chest and the other set of pipes connects to the cylinders. The piping for the inlet of the saddle comes from the steam dome and runs through the boiler. When the throttle is applied the steam leaves the dome, runs through the pipe and into the saddle to the steam chests.

The steam chests is a fancy valving mechanism. When the locomotive is shifted between forward, neutral, and reverse in the cab it is translated to the steam chest via some linkages attached to the Johnson bar.

Picture from Highball Sim
In this picture we see the linkage from the Johnson bar entering into the side of the steam chest. It moves forward and backward. When the johnson bar is in the forward position, steam from the saddle enters the steam chest and is applied to one side of the cylinder piston. Switch it in neutral and the steam from the saddle enters the chest and is directed immediately out the exhaust. Switch it in reverse and it applies steam to the opposite side of cylinder piston reversing the locomotive. As the locomotive moves this rod moves in and out in time with the drivers on a elliptical linkage on the axle. So it is alternating applying steam to one side then the other of the cylinder piston. So the piston is powered in both directions, one way then the other. During what we will call the opposite stroke, where the steam is applying force to one side of the piston the other side is opened up to the saddle to exhaust the steam from the previous cycle. This is the chuffing we get as the steam is exhausted on side then the other then back again, chuff chuff chuff.

Oh, I better add this now or we will be confused. So I said that rod moves back and forth. and I said it is attached to an elliptical linkage on the axle. Well that elliptical linkage is what is actually attached to the Johnson bar. So what happens to reverse the direction the drivers is the Johnson bar changes the relationship of that elliptical linkage and the bar sliding in and out. I realize this is about as clear as mud, Heck I am confusing myself just talking about it. It was also very hard to wrap my brain around until I watched video animations of it. So I am going to leave it here and just say that its magic and it works.

Once the steam leaves the cylinders it travels back through the saddle and into the smoke box where it is ejected out the stack. Remember here that not only is this just a good way of getting rid of it, but it is also acting like a super charger to draw more draft through the fire tubes.

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Phew, I think I gave about as basic an understanding of the main operation of the boiler and its relationship to the cylinders and steam chests as I can without getting too deep and making it totally incomprehensible. There are a ton of things I am leaving out. especially how that whole Johnson bar thing works and all the weirdness going on between the frame rails of the chassis on the axles.

Next, I will move on to some of the other gizmos and gadgets we find on our locomotive. I’ll work my way back to the cab picking up some of the other dohickies we see.

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The Dynamo/Generator.
Prior to the wide spread use of electricity on a locomotive lighting was done with lamps that burned oil or some other flammable. In the early days of electricity it was either fixed to poles or provided by huge generation plants. Even before the wide spread use of electric locomotives some smart person figured out how to make the dynamo or steam powered generator for steam locomotive use. This gave the locomotive the ability to use them fancy 'lectric lights. I am not sure if Pyle invented it, but the folks at Pyle-National surely perfected it.

The generator or dynamo is really nothing more than a steam turbine generator. Steam comes in from pipe that starts at a valve usually on the turret. It spins the turbine and is exhausted out a vertical pipe. That turbine is connected to the other side of the device to a generator. It spins the generator and produces electricity. That is.

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The Westinghouse Brake Compressor.

I got this photo from Mike’s Railway History in an article he wrote called “Stopping the Train” Go there and visit that page and you learn as much as you likely need to know about steam locomotive brakes. So much so I will spend no time explaining it I will just point out the pieces and give a very basic description of its use.

Westinghouse and others developed a positive air pressure brake system. The compressor has a turbine on the bottom where steam enters, spins a turbine and then is exhausted. That in turn spins a drive shaft, that in turn drives the air compressor on the bottom. This picture from the same website says it all as far as the plumbing.

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Now we will move back into the cab this will be another quick description. as we already discussed most of what is going on in there. But we did leave a couple things out. The Johnson bar

From Discover Live Steam.

Shifts the locomotive into forward/neutral/reverse.

The brake actuator

Picture from “Stopping the train”
An air pipe in and an air pipe out attached to a valve. Operate the valve to control how much air pressure you are delivering to the brakes and thereby determining how much brake is being applied.

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