With the current test builds, it is possible to set a door as EXTERNAL, so that you can enter through it when it is open, or to set a door as CONNECTING, so that if it is closed you can't go into the next vehicle.
I suggest a "boarding point", which would function like a permanently open EXTERNAL door, and also the ability to set the vehicle as non-corridor like if it has a permanently closed CONNECTING door. The first should be able to be combined with regular EXTERNAL doors, and the second with regular CONNECTING doors (so you can have an "end of train" coach that has a corridor on one end only)