Why trains ?

Put simply, the Rail Simulator 3D engine supports the movement of vehicles along a path. However, there is no reason why we should restrict this to trains.
Consider these fun ideas:

Cable Car
The cable car could be suspended high above the ground on a network of suspended cables. The cable car would move along the cable under the players control.
Canal Boat
There could be a network of canals. The player could move the barge along the canals.
Monorail
Similar to railways, but with a more spectacular viewpoint.
Tram
Similar to railways, but with a more confined viewpoint. This would allow very detailed cities to be built close to the player viewpoint.
Bus
This would use a similar environment to the tram network. Again, very dense detailed cities with lots of roadside furniture.
Roller Coaster
Again, similar to railways, but with a more spectacular viewpoint.