In the blog post, Simulation Part 2, a picture with a grid of transport vehicles was posted. Two electric trains and a diesel locomotive were seen which visually confirmed trains were a part of the game.
Actually, rail transportation was developed partially. One in-game trailer showed a train running down its tracks in the background and the cinematic trailer featured an overhead electrified train proving that Monte Cristo planned on adding trains to the game.
Both Passenger and Freight trains are included as means of Inter-city transport.
In the Simulation Part 2 blog post, it stated about "CityLinks" that ran around the city to enable transport links to neighbouring cities. If the player builds too many road links and not enough rail, the roads would be congested which suggests that freight trains were intended to be just as important, if not more important for freight transport, as trucks.
Trains were planned for Cities XL 2011 but with Monte Cristo's bankruptcy, the existing trains, tracks, and station have only been released as part of eyecandy mods, such as GlobexCo's Unlocker mod. There used to be a fatal error or crash whenever these tracks were dragged to the intercity rail CityLink but that has been fixed. Currently trains appear in the official game but these belonging to the Metro system.
There is a big transportation mod in the pipeline, though, named Network Expansion XL (NEXL), which plans on releasing the train system in its intended role as an alternative Inter-City form of transportation. The ETA for the mod is not yet clear, but it is expected that the new modding tools will go a long way towards opening the possibilities for modders, and thus NEXL release might be delayed to accommodate the new options.