Since you posted this in 'Track and Trestles," I assume you are asking advice on that subject.
First, it depends on the scale you intend to use. If you are planning on modeling 24" gauge prototypes in 1:20.3 (15mm=1’) scale, 24" gauge is 1.18" or 30mm. I have a 1:20 scale construction railroad that takes a little liberty by using 1.25" (32mm) gauge track. That gauge is what is used for 0-scale standard gauge models. In 1:20.3 scale, it equates to ~25.6" prototype gauge. Close enough for me, but some purists have chastised me for terming it “2’ gauge.” In fact, early railroads were frequently built to gauges other than 24", sometimes to get around laws specifically aimed at “24” gauge." Here in N. California, there were a couple of 25" or 26" gauge operations.
That choice of modeling gauge allows me to use a lot of off the shelf stuff, including 0-gauge mechanisms and wheels. I am not interested in hand laying track, so I purchase PECO SM32 track that is commonly used in the UK for 24" models in 16mm=1’ (1:19) scale. The rail is code 200, and the tie size and spacing provides a lighter look without sacrificing durability. Some of my PECO track has been out in the weather for over 5 years with no detrimental effects.
I have rebuilt several of the too small Bachmann box and flat cars to represent tool cars and other utility cars for my 1:20 scale 2’ gauge railroad. I also plan to get a couple of the IP Engineering critters such as the Lister Diesel and petrol units to power the construction operation. They are also 16mm=1’ scale, but I defy anyone to find that extra millimeter!
If you intend to model 24" gauge prototypes running on 45mm track, the correct scale ratio is 1:13.5 or 22.5mm=1’. Typically the very slightly incorrect scale of 1:13.7 or 7/8" (22.2mm)=1’ is used. There are a number of modelers working in this larger scale. I’m certain some will chime in here.
As Bart asked, what information are you looking for?
Happy RRing,
Jerry