After everyone climbed into the train cart they noticed that Maggie was missing but no one went back for her assuming she should be fine. While Maggie was struggling to climb on the cart she fired a few gunshots. That should've made it certain that someone was in trouble. Presumably Maggie.
In the opening shot from under the train it is clear there are no zombies under the carriage but immediately after, in the next shot, Maggie's leg is grabbed by a zombie under the train.
There was no reason to leave Gage to die. The group had just come through that door themselves so it definitely wasn't jammed. There was plenty of time to open the door, let him in and reseal it. In fact there's no clear reason why he couldn't just open it himself as the others had done earlier.