Light rail transportation (trams) was planned in the original Cities XL. It was never officially confirmed but there was a tram model included in the game, which means that Monte Cristo was working to implement trams in the game. Most likely it was to be part of the Multiple lanes feature where the tram line can be on a median strip or shared with cars on the road.
Trams were mentioned on a fansite's article about the game. Roughly translated from French:
|“||As everyone knows, transport management is without doubt one of the biggest headaches offered to city-builder players, and Cities XL will be no exception. Each of our workers will be subject to the effects of a tolerance / preference for different means of transport. For this, it will obviously require a road network but also the need to develop alternatives, such as railway stations, bus services, or a multitude of subway lines (with management of their connections), and trams.||”|
— Gamekult article
As we all know, however, Monte Cristo never got around to releasing any of the ambitious transportation features but Buses. Cities XL 2011 has the closest thing to Trams with the Metro system though subways are not trams.
In the German community Cities XS were some train projects planned. An rapid transit S-Bahn/RER system by Oliver and a tram system by haltendehand. The German modder haltendehand modded a working tram beta version for Cities XL. It replaces a bus and is a bit buggy. It is not an official feature and is exactly working like the bus. Because of some troubles around the possibility of crossings and of coupling trains both mods never were released. The closed beta for RER system isn't open for public anymore, but the tram beta can be still downloaded here (German):