As nobody seemed to be able to reply
I tried something myself... Now I can make a 2 way train but it turns when it has to do the comeback way (I wonder if that "Reverse branch" flag in the path_track has any use...) and turns again when has to do the "Go" way...
For everyone who wants to know how to do it there it is:
-Make the func_tracktrain.
-Make the path with path_tracks.
-Make the reverse path with path_tracks and link them together (link first and last of each one).
-This way the train will do an infinite movement among the two destinations... To make it stop simply trigger the train in the path_track you want the train to stop. It will stop itself. This assumes you have a way of triggering it back to work, of course.