Here is the subway system aside from the above ground services which are in purple in the previous picture.
I was just going to suggest this. It has a great mix of heavy and light rail. It has both tunnels and plenty of outdoor scenery. Also, every statiion is unique and interesting. The total length of the subway system is about the same as each of the routes they have published so far. The tricky part might be the street-running sections. TSW doesn't have a ton of car traffic, so that sort of mix might be a challenge.
The Haverhill line has 14 stops including Reading Massachusetts which would be awesome to drive through on a Train Sim world.