Oh yeah, I love trains too!
But it would be a challenge to add them without spoiling the gameplay. For now, Matt's idea seems to be the closest one to what I would like to see. NPC-driven trains, which are circling around the whole San Andreas, and SF trams would be a nice addition to our game. They can be used for temporary (or permanent - in SF) camping, or just for fun, but they would definitely be a nice feature of our server.
The only noticeable obstacle I see now is objects placed on rail tracks like SF terrorist spawn vehicles. Although, as only the tracks which are closer to the depot are used, moving a few cars aside wouldn't be a big deal. But it should be done before even thinking about the trains anyways.
+1 to this suggestion.