Well, I have. The Razor trains are about 3-4 times as high as they are wide, lack most standard train features and tend to travel very, very fast. Probably crewed by something non-human, too.
A Combine railroad would probably be best kept as a single small guardpost in a post-apocalyptic setting, lots of broken and rusty engines, with a few "commandeered" ones. All trackside industries would be under Combine military control, with the surrounding countryside laid to ruin. Operations-wise, it'd be a fairly standard layout, with switchers dealing with industry, heavy freight trains being assembled and taken apart, and Razor trains speeding along the mainline (I don't think they stop outside Combine citadels or cities).
Geographically, HL2 is very ambiguous, but there's a definite East European flair to it, so something from that area would be appropriate regardless of being anachronistic, like big clunky Soviet diesels. It'd be about the overall atmosphere of the setting, not any idea of historical accuracy. And of course, in some corner of the thing, a rogue scientist in an orange hazmat suit.
As you can see, I've given this idea some thought.
Edit:
Looks like the visual designers of the Razor Train were insipired by reality after all:
http://www.skyrocket.de/locomotive/data/nw_te1_jawnhenry.htm. It's big, blocky, ugly as sin and looks like it could pull a small country behind itself.