I think one of the worst people to have standing in front of you in a crowded train is someone who brings on a big rucksack and doesn't bother to take it off, and then slowly backs into you, encroaching on your space with the rucksack.
If you're going to walk through a crowded place holding hands and neither of you is handicapped, then you're just being inconsiderate to anyone who's trying to get past. And if you're going to walk really slow, at least leave room for people to overtake you.