Here's a possibly fun concept for the new multiverse. You mention trains, so maybe one train has a dimensional incident that causes it to be stuck flowing through bizarre spacetime whatever land that causes multiple branches of reality on the train to occur. They almost all appear to lead to the train ripping apart and destroying the main world that our multiverse takes place on, but the people on board must find out a way to get one branch of reality to work out where the train goes back to normal space and they disembark onto the world with an awesome story to tell. This idea is inspired by an awesome anime called baccano where it largely takes place on a train and they keep repeating parts of the plot and revealing more of it from different character's perspectives every time. In our case some aspects get redoes as different reality versions of themselves take the lead to try and fix the doom of the previous one. 

This plot could actually turn into a massive roleplay and different battles playing out by people on certain sides, as we know at least someone on board wants the train to dimensionally explode everything. They would also be talking, plotting, etc. without always having to physically fight. If a character dies then you don't need to worry because we can redo that plot point with the next dimension taking over, but all of this still matters because the characters are learning so it's not all for nothing. Plus we can all have a lot of characters. The train will be huge, with some cabins even having gardens and ballrooms and stuff.