I had a SS on the tube the other day.
The platform was quite busy, when the train pulled in I was at the front by the door. The train was also quite packed but a lot of people get off at this stop. When the train stopped I was still in the front of everyone, then stepped to the left out of the doorway so people could off the train first. SS was closer to the right side of the door, and stayed in the way of the people getting off. Most people were coming through the right hand side, whereas there was a slither of space down the left hand side where I was. However you're supposed to let people off first before getting on, thus I was not about to push through all those people. Instead, SS steps in front of the people getting off to go around, push through me and then go through that space.
Because I seem desperate not to get back to writing my novel, I ended up drawing this incase I wasn't clear. I'm red, SS is green.