TL;DR
A hotkey to stick a locomotive to a train signal as it sticks to a station.What ?
When you place a train near a station it sticks to it allowing neat alignment of inserters and other stuff.But when you build your train network or create intersections you sometimes need to adjust length of the rails and positions of signals accoring to train length. So it whould be usefull to be able to stick trains to a signal to allow alignment of other parts of rail network. I think simple hotkey would be enough.