Back it in
When boarding a really packed train in the morning, it may look like there is absolutely no more space, with people already flush against the entrance way. However, don't be fooled, as there is still space in between each person. They simply maximize the amount of space they have at the moment, so they won't be offended if you make your way in there also. No one really talks to strangers during the morning rush, so it is best to approach the full train with a slight bow of the head, and then scoot your way into the train back first, and then even support yourself by grabbing the top of the door frame. You basically just have to consider how to make the least "offensive" contact with the other people on the full train.