Just that: the Q key (PC/Steam) does nothing. Moreover, the footplate on this train model isn't interactable, so you can't even reply with the mouse. Basically makes the train un-0drivable, unless Sifa is disabled. This is definitely a new bug introduced with the TSW3 port
Just to isolate this bug: BR422-SIFA works fine on PS5 with Keyboard "Q" & Controller. Tested at SKA, RRO & HRR. Must be a problem of PC-Version.
There is a longstanding bug with this train, for the sifa button to work you need to stand up and sit back down after spawning.
I tried again a few times without standing up and it still works fine with both keyboard and controller.
Do a "sifa" search on the technical reports forum and you'll see that they've been ignoring sifa issues with the 422 and 425 for over a year. I'm currently kinda into HRR and thought about reporting it but looking through all the threads it started feeling like it'd be a waste of time.