how to return a reference to an object?

[Gribouillis]

f i make a different function to do the increment, i can make work. but i want to have it all done with one function that simply returns the object by its key (which_train).

Please describe the whole problem with the cars and the trains and what each function should do (detailed arguments, effect, return value). You make us guess both the problem and the solution. Frankly I still don't understand what you want to do and why.