Agree that it could be clearer. The idea of the '?' is to let you enter a location without the hassle of having to save it first. Useful for times when you want to go someplace, but it's not common enough to warrant saving it
Nitpick aside, this is a beautifully simple interface.
Needless to say, but well done!
I know where home is but I still don't know whether or not I have to run to catch the next train.