Um, OP, let me get this straight
you're saying that where you are, a packed train is one that has no seats, but does have space between seats, space in aisles etc? (To me, in London a packed train is literally a sardine can so no space to even see a mum struggling to feed a baby).
if that is correct, it is even more of a mystery why you, and the others in front of you who refused to step off the train, chose to stand in front of the doors.
your observation that no one folds buggies on trains is quite right - well, not "no one" but hardly anyone. I have been known to tell people to fold buggies if they are looking blankly at a wheelchair user, it's not on.
I haven't been brave enough to say "pack up that buggy to make space for 2 more people standing" I must admit.
And I still have no idea what, if anything, your friend was doing to help. I'm actually surprised no one shouted FFS Move! at the pair of you (and all the others blocking the doors) if the aisles were free.
I don't know how old you are, but in my day (!) it was quite normal that a London bus driver wouldn't even let you on unless you were carrying the baby in your arms and were prepared to put the buggy, folded down, in that section on the bus. So we've actually become much more friendly towards buggies, which are often so large I worry people are planning to send their toddlers to war, but that's another thread.