Subway tunnels are likely to be as narrow as the builders can make them. For those with a single track, this is probably means being about ten to twelve feet wide and about as high. Where there are two or three tracks, the width is doubled or tripled, but the height stays the same.
Most such tunnels are likely to have concrete walls and ceilings, or brick in the oldest examples. The floor has the familiar metal rails laid on wooden slabs, laid on gravel or shingles. There is no lighting. Every 200 feet or so, there’s a little niche in the walls, allowing maintenance crew to get out of the way of trains as they go past.
Every turn, there’s a 2-in-6 chance for each track that a train comes along. You get a minute’s warning (6 rounds) if you’re listening, for it, or 30 seconds (3 rounds) if you’re not. The train goes too fast to run away from: one second it’s out of sight, the next it’s hit you.
Getting hit by a train does 3d10 damage at the point of impact, and then d10 damage each round thereafter as you’re dragged along. Getting out from under the train will require something impressive.
In single-track tunnels, the train pretty much fills the whole cross-section of the tunnel. You can’t step to the side and shelter against the wall. You don’t get a save to jump out of the way; there’s nowhere to jump to.
If you can get to one of those niches in the wall, you’re safe. At the point that a train is coming, the next niche is d20 x 10 feet away. At a sprint (combat speed) you might be able to make it there, or you might not. Sprint in the wrong direction, though, and before you get there, the train hits.
You could lie between the rails and hope it passes over you. That probably succeeds. For each section of subway, when the players first try this, roll a d6. On a 1, the model of train that runs on this track doesn’t have enough clearance for lying between the tracks to save you, and instead everybody under there gets hit and dragged along.
For two or three-track subway tunnels, roll a d6 for each to see which tracks have a train on them that turn. If all of them have a train that turn, then all three arrive simultaneously and, as with single-track tunnels, there’s no room to get out of the way. Otherwise, stepping out of the way onto a track without a train on it is a trivial matter.
Just for fun, you might want to randomly determine which direction the train is coming from when it hits you, and so which direction your mangled carcass ends up going in. People being hit by trains in unusual circumstances (such as heavily armed individuals, or ones dressed in weird culty robes) might increase the party’s unwanted attention score by 1, too.
In abandoned subway tunnels, of course, you don’t have to worry about any of this.
Encounters in subway tunnels are incredibly rare: roll once every five turns. Most of what you encounter will be small, incorporeal, or otherwise able to avoid Death By Train.