Yeah, that's the thing. I already spent a couple of pages on TL1-2 naval warfare, including some untested wind, rowing, and ramming rules, so I don't want to go too far into the train rabbit hole, just enough to keep it plausible and playable. Working out grade issues, stopping limitations, efficiencies and such in detail is not the focus of the book. We're never going to have high realism with Speed Bands and Spaces.
Never been to Japan, or on a fast French train, but it has been my experience that, yes, US trains often stretch past 100 cars (especially if I'm at a crossing) with multiple locomotives and European trains seem to be very short and single locomotive. I could just limit it by setting a maximum speed for the rail, so even if the locomotive can go "Very Fast" in principle, it would be limited to Medium by the rail - spend more on the rail (or have a higher TL) and the train can theoretically go faster on more expensive tracks (exceed the speed limit and start rolling for 'bad things')... plus then there's maglev and all that.
Easiest fix for that is to think of locomotives like gearing (and it's an apt analogy considering how they actually work). One locomotive can pull you X number of cars. Say it's rated for 50 cars at 100kph. Even without the 100 cars that locomotive may not do more than 100kph because it's not engineered to run fast even when running light. The length of trains is determined by desire - European or Japanese could easily run 100+ cars like American ones do. However they choose NOT to work that way. Just like US trains could increase there average speed in the high double digits (or have faster passenger trains), but US railroads have chosen saving credits over investing in higher-speed infrastructure. Which means a C449W 6-axle locomotive is built to pull a lot of cars, but not fast since there is simply no environment within which to do that.
Rails are built by weight-class (the sturdier the rail the more it weighs). At points in US history there were railroads that literally laid rail on graded dirt in order to complete routes and get land grants. No ballast or any other effort to make it last was made, as the thought was to come back and do it later - if at all.
On frontier/poor/ramshackle worlds you are going to see all kinds of variations for rail. Industrialized or populous worlds should give you more regulations and railroads that invest for the longer haul. You'll find examples of neither on any world as well. But that thinking is a good rule of thumb to give. Oh, and you'll also find in some cases railroads that are built to very good standards even though their surroundings would not necessarily lead you to expect it (the reversal is true, but so far it seems to be less likely). Historically you can look at the British who built rail infrastructure that is little changed in parts of the world, and you'll find many examples of similar things done in the past by US railroads.
Maglev, conceptually at least, is basically the same as standard rail. Unless you have steam locomotives, all trains are going to be electrically powered. Diesel carry their own generators, and electric use external power. You can get into other fuel sources (gas, hydrogen, even fuel cells) that all do the same things - generate onboard electricity to power the wheels. Don't forget you have rails below (classic train tech) that includes maglev. And you also have trains that can be suspended from above (suspension railway). Suspension rails have never really taken off, but you may see them in urban areas (I think Babylon 5 had that type i one episode). Oh, and there are cog railways, but again, probably going too far down the rabbit hole for most.
Unless you are a train nut most people will not even consider trains ('cept maybe to consider them nice hijacking targets.
Oh, will we be able to build the Mammoth car from the classic Speed Racer episode? Or the land truck built by the US Army in 1950? They are cool, though turned out one of those cool factors that never amounted to anything actually useful. Thank god for taxpayer $$ and animation!
