Renderosity has a Monorail from RPublishing that fits into Neo Station, so it might work for an updated Parkside. (Though I’d resurface the train for that, myself.) It would look a little odd in head-on renders, but the track gauge may be right—it works in Neo—so it might work in Parkside.
They’ve also got the IN Elevated Subway for sale; I don’t know if the cars can fit into another set or not.
Aemi1970 has some items at ShareCG that might work, but again, resurfacing might be needed to make it work in Parkside, and a lot of editing IS needed. (Also, fair warning: Aemi1970’s catalog varies between the mundane and the Very Very VERY Adults-only—I’m a bit surprised that ShareCG allows that.) They’re some earlier work, and I think Aemi1970 had not yet figured out how Poser-style paths work, so the paths for the geometries and textures for all the PP2 files are well and truly borked. I was working on them the other day myself, and I just gave up at one point, because they’re such a mess that it’s very easy to overcorrect and create an entirely different mess. I may try again later.
mph-com has a bunch of European styled trains in Renderosity free stuff. Haven’t really done anything with them yet.
If you’re a member of Poserworld, they’ve also got a train car that might work, but it is just one car.
If you wanted to try something a shade more vintage looking—tourist attraction for East Park, maybe—you could resurface the Parkside Passenger Car and Parkside Freight. (I wish someone would.)
(Purely a side note: I do wish people who make train cars for passengers would divide the material zones lengthwise along the car, so you could hide half of it to work on a scene. For all the cars I’ve mentioned, except maybe the NPH cars, while they all have lots of material zones, you wind up having to turn off a lot more than you probably want, just to see what you’re doing.)