as I mentioned in the problems forum the diagonal cliffs are sometimes jagged(depending on which way they go)
an Undo button for the level editor
A random terrain generator would be great
the ability to import a bitmap as the terrain map would make it easier to build intricate levels (different colors = different terrain heights
Tunnels would be fun.
If the train could turn 90 or 180 degrees you could do many things with that(could be a texture for left/right turn)
maybe have multiple planes of water (like a #### or waterfall)
The ability to overlay a large bitmap on top of all or part of the terrain.
tree's, houses, roads, cars, buildings, boats, people....various models you can place in the levels.
backgrounds-skies, clouds, pictures....
Night-time mode- would need to place lighting in the level design and have lights on the train and on the tops of the bridge.
other eye candy like weather (rain, fog)