Slightly deviating from the 'main' music, a system for in-game music could just utilise music on the local computer.
Creating a shortcut to a music folder from "My Documents/Transport Empire User files/Music" would allow TE to search the contents of the folder (and sub-folders?) and add it to a list of music to play in game. Futhermore, like LoMo, the songs could be played relative to what year it is in-game, via reading the ID3 tag for example.
Unfortunately, you are limited by what is on your pc, and your choice of music, so this could be added alongside the game's own music.
EDIT:

Back on the theme tune, my music teacher does some composing, shall I ask him to do a little tune? If so, suggest how the tune should go, i.e. Upbeat, swing etc.. and I'll ask him when he comes round on Wednesday.
The occasional look back at your past can teach you a great many things...