1) More texture for the station and metro stop. I know it sounds trivial but theotown only has one type of station and subway stop.
2) Draw the lines. With the buses you can and with the metro you can't.
3) Ticket generation. Why can buses do this and the subway can't?
4) Make the subway and the elevated railway the same thing. Maybe it requires a lot of work but I would like to be able to build the subway half underground and half elevated. The subway tunnel must not require diamonds.
5) Texture for subway trains. Too bad that I can't see them.
I hope that these suggestions will be considered.
