As much as I love my recently built train, the train track it's running on is... well, it's an endless string of S bends, and not really due to terrain either.
It seems that trying to build a straight track by extending an existing track is very difficult, as the the "end" of the track segment has auto-assigned direction by the build tool.
To build a straight (or straight-ish) track, one must first create a bit of straight track WITHOUT connecting it to an existing segment and only THEN connect it. This is very counter-intuitive.
Additionally, I dare say that the max length of a single placed track segment is somewhat low, given the "thickness" of the track and the above "follow the path" logic.
I'd suggest the mouse wheel could be used whilst placing a track fragment to rotate the ending, thus giving the player more control over the desired shape of the track. This could be handy both when extending an existing track as well as building a new one.