By jumping off a bridge at CP 216 from the cab of the ACS-64, you can get out of the map and run east. Eventually you'll get to a point where you can hop onto the tracks and explore CP 217 (the interlocking east of New Rochelle). You can continue east up to a point where there's a wall blocking you. After circumventing the wall, you will encounter a building that has a collision mesh that you can't pass. The only way to get through the building's collision mesh is to wait for a train to come on No. 4 track and have it push you through the building. The train disappears right after it passes through the building. It's at this point where the ACS-64 consists spawn and despawn. You can continue walking down the tracks for a while and will eventually reach a point where the tracks end. If you walk any further, you fall into oblivion. It's definitely worth a try. I'm going to attempt a similar stunt west of Newark Airport and see how that goes. Here are some screenshots. Subsequent posts will contain more photos of the boundary.
Yeah the tracks that go off into infinity are the way TSW does spawn/despawn portals. Not a bad approach though, it allows you to get a train out of sight before it gets despawned.
These shots of NEC's "twilight zone" are certainly interesting, as is the fact that although spawning a train from a portal in TS will keep your scenario from being accepted by Steam Workshop, the TSW developers have even landscaped their spawning areas. I especially like the shot of that fan of tracks rising into the air.