[Tyhm]

You can purchase tracks at a shop somewhere.
Lay only the ones you have (turns are more expensive than straightaways! And crossroad pieces are way more expensive!)
You can only lay a railroad piece over an existing railroad piece at the end (in case you find out you're running right into a mountain and need to turn), or connecting to a "live" railroad piece. A railroad piece is considered live only if it connects to a station on the other end. Simple linked-list form would work for storing that....this piece connects to that piece, the end of this piece is a station....

That way players can't do huge loopy things unless they spend a LOT of money on it
They have to connect to at least one station/preplaced track

Kinda like Pipe Dream....anyone else remember that game?