Since no one mentioned it, the sine qua non for the railroad scene still is Wathers (http://www.walthers.com/)
Caution, you will probably need a refill for your beverage. Probably a good idea to open wiki in another tab (if only to chase down the scale notations (like On3 or HOm). But, also to keep track of gauge--which is the spacing between the insides of the rails.
On gauge, most "big" railroads use a rail spacing in the 4'-6" to 4'-10" sort of size as a 'standard' or 'national' gauge. For the US (UK/Commonwealth too, IIRC) the spacing is 4' - 8-1/2"an odd number held to be the width of Roman wagons based on the width of the draw-beasts used. As nations formed and alliances rose and fell in Europe, there were strategic/political reasons to vary the gauges (mostly to deny access of other nation's railway equipment).
Except . . .
In wilderness and exploitation areas the heavy rail equipment was less needed. So, "narrow gauge" stock was used. Now, the wheels, couplers, brakes, etc. were all the same size as mainline equipment, but the rails wound be 1m/3' wide or 60cm/2' gauged--this allowed using smaller sleepers/ties to support the rails. and allowed slightly narrower corridors cut; and significantly tighter radii for curved track.
Where this gets beyond esoteric is from the view of a model manufacturer. What to model? UK stock; French? German? Eastern Europe? What to make? Locomotives, cargo cars, passenger cars? Standard gauge or narrow.? If narrow, which narrow?
Here's a bit more to muddy the water. In current US locomotives, they break down into 2.5 groups. Which are 4-axle, 6-axle, and everything else. There are fewer varieties, probably only a dozen, down from the two dozen or so styles of locomotives a mere 40 years' ago. If we include steam, the variety is near endless. The European stock is as equally varied.
However, there are kits of all sorts of stock--they are just in HO, O; some N, and a smattering in 1 gauge (1/32). Largest variety is probably in HO. The boxcar kits are fascinating, the wooden parts are marvels; the working parts a study in micro engineering. I caught a bit of this bug for having a college roommate who was a very serious model railroader--he was more a builder than a 'runner" though. Not too much worry about fuselage seams (except tank cars)--but getting decals to no silver on actual wooden car sides is a task.
As noted above, there are some very serious brass kits. They are spendy, and highly intricate, and all the ones I have seen require a deft touch with soldering. But, it's hard to beat actual metal when modeling metal. Being able to .use chemical blackening instead of paint is unique, too.
Our modeling maestro Pawel has a long thread on his build of a 1/32 WWII German locomotive [dang it, I cannot find it to link; I want to remember it being in Armor, but, it may be here in Autos].
The US Navy's SeaBees used 2' narrow gauge across much of the pacific. It turns out HO rails space right at 2' wide in 1/35 (other than the ties are the wrong dimension and spacing. But, a person could get HO working gear to cobble up some narrow gauge wartime models.
Oh, and to finish my ramble, one other problem in building static kits the manufacturers face is the length of trains A train 3000'/1 km long is not terribly uncommon. A narrow gauge log or ore train can still get 5-600' / 200m long. At 1/48, a 200m long consist will be 4 meters (plus an engine) long. That's about 14' of track to lay out. (or about 2km in HO scale).
Hope that did not over-muddy the water.