It's a recent Cartoon Network series that premiered last year. It was originally one of those rejected pilots that then got uploaded to Cartoon Network, saw MASSIVE fan response and then got greenlit at a later date, so a lot of people were excited for it.
Basically, it's about a young girl Nnamed Tulip, who runs away from home after an argument with her divorced parents, only to then wake up aboard a mysterious train, and with a glowing number printed on her hand. Each car in the train is its own little strange world operating on its own little strange logic, and each of them requires her to do some sort of task before she can get out and move on to the next car. All the while, the number on her hand changes depending on her actions. So she tried to find a way off the train, or to find the conductor of it for fear of what happens if her number reaches Zero.
Fans were really excited, only to then find out the series was a 10-episode miniseries at 10 minutes each (a la Over the Garden Wall from a few years back)... and then the Season ended with a "See You In Book 2", indicating that the series will run on a miniseries format, seemingly focusing on new characters each season. So they aired Season 2 over the course of a week, two episodes a day, recently.
It's a VERY good show, a rather intense children's horror/adventure/comedy, dealing with how to handle various personal issues.
It's DEFINITELY worth seeing, especially since one whole season is basically the same length as a fairly standard movie.