The only service vehicle in the UK that I am aware of that runs on tram lines WAS used by London Tramlink. Oddly being a former DB Netz 53, it's now at Crich. I think we use rail/road Lorries and Cherry Pickers now for trams in the UK.

As for light rail, off street running sections, sometimes locomotives. Although Manchester Metrolink once used a Austerity Steam Engine to run a maintenance train for some reason.

The Franfurt transit has also some rail-road vehicles

I see. It's easily for them to be used.