Double tracking

Agreeing with Cengro on this, but more comments. Double tracking makes a huge difference in the capacity of the train network. Building double tracks is a royal pain. Especially in the curves. To do this right, make an option in the track building menu for 'double'. User selects a section of track, presses the button, and a matching track section appears next to the selected section. Doubling a track to the outside of a curve is just a matter of adding a few small sections to even them out. The difficult part comes when doubling a curve to the inside, because inner curve may be too short to be formed. So throw an error and force a track redesign by the user. But being able to quickly double an existing single track without a lot of monkeying with the layout is a real quality of life improvement