So there is a Citadis model that does power injection

The way I understand it, it an inherent feature (it should be with IGBT inverters), and use depends on whether the tram line power supply was made suited. With some more search, I found other examples of active use (Rotterdam, Sydney, Basel, etc.), so it may not be uncommon. Recovery rates are usually low, but in Basel, a phenomenal 41.6% was achieved with Siemens Combinos in a test. The latter link also gives you kWh/km figures (page 8).

_{*Lunatic*, n.
One whose delusions are out of fashion.}