Subway salmons refers to the one or two people who insist on going up a stairway or down a stairway (usually after a train arrives at a subway stop) while all the people leaving the train are going in the opposite direction.

地铁三文鱼指的是地铁站里（经常是地铁到站后人大量涌出时）坚持逆着人流上下楼梯的一两个人。

Example:

"Of course there were subway salmon when I was late for work and I was trying to get out of the subway in a hurry ... Dude, couldn't they tell the train already left?"
“上班迟到想赶紧从地铁里出去时总有‘地铁三文鱼’……老兄，难道你看不出地铁已经开走了吗？（还挤个什么劲啊）”