I know I've seen the conductor go into the door midway in this train, opening it with a key. But I have to admit I don't actually know that he was able to get from there to the other half of the train. Maybe he was just hiding out because he got tired of all the bowing!

The walk would be a shorter at Shinagawa than Tokyo station, but at Tokyo station there are escalators (even some elevators if you can find them) to cover most all the up and down. I remember that there are escalators up from the shinkansen at Shinagawa but I am not sure that there also ones that go all the way down to the other tracks there.