Large Scale Central

Steam Locomotive Anatomy 101

Just a few last bits of stuff to finish the article off. Most locomotives have a bell. Bells can be either hand rung with a pull cord, or mechanically operated with air pressure.

Inside the cab you will have some means of actuating that bell so either a cord or a valve handle. If a locomotive has gravity fed sand domes there will be a lever that attached to the base of the sand dome and extends back into the cab through the front wall. You will see a cord for the whistle that either goes through the front wall out to the whistle on the steam dome or some whistles are on the cab roof if the steam dome is inside the cab and the chord will come through the roof.

On the floor in the cab is sometimes a lever that you pull up or push down that is connected to the ash pan. The ash pan is nothing more really than the floor of the fire box. There are doors on it that drop down and dump the ash into a lined pit between the rails.

On a vacuum brake locomotive you will see a little different set up than the positive pressure brake systems like the Westinghouse. The brake handle on these almost looks like a hybrid between the Westinghouse brake lever and an injector. It has an exhaust pipe that exits through the roof of the cab. That device is called the “ejector” and looks like this

Amazon book on Eames Vacuum Brakes

i have a fondness for the Eames vacuum brakes. One because the Coeur d’Alene Railways and Navigation company used them exclusively for their short existence and because they are just different. Unlike the Westinghouse system they are steam actuated and use a venturi like an injector. You pull on handle that is horizontal and this “turns on” the brakes by applying steam to the venturi creating negative pressure. With that on then you can pull on the vertical lever which regulates how much vacuum is applied to the brakes and thereby determining how much brake is applied.

I think that’s about it. I hope it helps. If you are going to want to actually model this stuff I highly recommend downloading and reading and following David Fletcher’s build article. There is also a lot of diagrams that show the various stuff. And I will always try and answer questions.

I hope you enjoyed this and found it useful.

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This is awesome, Devon. Both the descriptions and the pointers to more info. Mucho thanks!

I do have one nagging question: it seems that there is a throttle but there are also multiple “stops” on the Johnson bar. Is the throttle just on/off? Or is it a fine tuning lever given a Johnson bar setting which is kind of the equivalent of gears in a car (without the gears of course)?

Or do I not have a clue what the relationship is between those two controls?

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While the throttle and the Johnson bar work in concert to make the locomotive go, they are independent systems. Really no different than a car with an automatic transmission. You can push on the gas all day long and it will rev the motor or slow it down. Doesn’t mean the car will go anywhere. The throttle controls the engine not the transmission. Conversely, the transmission can be moved from forward to neutral to reverse and if the engine is off it wont go anywhere. The Johnson Bar runs the transmission. The notches on the Johnson bar just hold the whole system of linkages out to the steam chest in a particular configurations. You sorta can control the speed with it but not because you are giving it gas but because you are riding the clutch letting the transmission slip. The Johnson Bar can be positioned to either fully engage the steam chests or some variation in between where the valving system is actually a little bit out of time with the drivers and lets the steam exhaust a little bit causing a sorta slipping action which does effectively control the speed. Just like letting the clutch out slowly you don’t just slam the Johnson Bar “into gear” or you will get quite a jolt. So you “notch it (my term)” more and more as the locomotive starts to move until its fully engaged.

now once it is fully engaged comes the throttle. It has no notching. Think of the lever on a standard regular old ball valve. The throttle lever is connected to that handle ( this is way not what it really looks like) and you move it back and forth to go from fully closed to fully open or anywhere in between.

Does that answer your question

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This is what it looks like inside a steam dome

Mid Continent Railway Museum


That lower picture of the cut away shows it pretty good. The throttle lever is connected to that rod heading off to the left, which in turn is opening and closing the valve inside the pipe. You can see where steam would enter that pipe at the top and then be “throttled” by that valve and then it travels down the pipe heading off to the right and into the saddle.

In this picture you can see the hole where steam exits the steam dome and enters the throttle valve mechanism.

Mid Continent Railway Museum.
If I have it right the throttle lever is attached to the bottom of that linkage. as it is pulled on it must hen pull down on a plunger closing off the valve. Push on it and it pushed the plunger up and opens the valve.

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Great Steam Locomotive 101course your posting, good job. Isn’t it interesting where a curious mind will lead a person?

Is that Dave Taylor cause all the old guys look the same to me?

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It was kind of refreshing to write it. It’s been awhile now since I was asking these same questions and I realized I started taking it for granted. Forced to explain it made me reexamine what I thought I knew.

It does sorta look live Dave

So all this started because Jim wasn’t sure what a Johnson Bar was or do you guys just like saying “Johnson Bar” ?

I just like saying Johnson bar

I just like saying Johnson

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I rest my case… Perry Rooster Mason

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So here’s what interesting if you look at the picture of the Johnson bar you see a series of notches. Both forward and reverse. I count approximately 16. Guess how many notches a modern locomotive has? Exactly 16 ( 8 forward/8 dynamics).

Funny how something simple like that is from years ago. But it makes sense. If your a steam engineer learning these new diesels than you would already be familiar with 8 notches applying power or slowing down.

I’m not sure if you could have used a Johnson bar in “reverse” to act like a dynamic break but maybe? I’ve never heard of that before.

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That is interesting ( you are a little sick for noticing that) but I would agree you probably are spot on as to why a diesel is the way it is. Transition

The only problem with your theory is it’s not than but then. Ha from the guy who can’t spell to teacher boom

Hey all my students already know that Mr. T can’t spell. That’s why they make spell check. :joy: Oh wait that was a grammar mistake.

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