You got the same issue if the spidertron is standing on top of a normal locomotive, or a car.
But I will look into it. Maybe I could not create the motor car if the spidertron is selected.
A configurable shortcut has the following problem: if you press <other key> to enter the motor car - do you also exit it with that key? And also exit it with <enter>? Now you have two buttons with the same functionality... etc...