Anyways, it's possible to build in TTO/TTD what you could call a diagonal bridge over water. Shock! How is this possible? Well, I found this out while messing around with the game.
Find a body of water you want to make a diagonal bridge in. Extend the track "into" the water (destroy the terrain near the water and place the track before it's restored, otherwise you'll get a "Can't build on water" error). Now, raise the terrain on the water so that it looks like you're forming a small diagonal river between the tracks. See the attachment for an example. Now, just let the train go and behold, it moves over the water.
This also works for road vehicles, though since they technically don't have diagonal roads, it's a rather slow method to move around since they'll keep "zig-zagging". For trains it's great though since there's no speed limit, even if very expensive. But there's no price for efficiency
Some might consider this a bug, but I consider it a hidden feature to be able to cross water diagonally since it's possible in both TTO/TTD with railway, monorail, maglev and road. Also, if you try to change the terrain while a vehicle is going through you'll get a "Train/road vehicle in the way" error.
So, enjoy!
Thanks for the tip-off. 
