I just stumbled above depots when searching for the object type of trolleybus catenaries and just remembered the bug depots not showing info window even if enabled in the dat.
I had a quick look at it and saw that depot_t overrides the show_info() method (not working) whereas signalbox_t does not override it (working).
However, I guess there is more to do than using parents implementation of show_info.
This is just a reminder for me or anyone who wants to take a deeper look at it, as I did not yet create a bugreport for this.