Access for disabled people, elderly people, cyclists, anyone with a dog, or anyone with luggage.
In London maybe one eighth of the underground has step free access: In Berlin almost one eighth of the S Bahn and U Bahn (overground and underground) hasn't got step free access! The lifts are smooth to stop & start, and large enough to fit a bike, or a large pram as well as a few more people. Most of the platforms have no gap if they're at a "barriere frei" station and v minimal height difference (maybe a couple of inches at worst, but nowhere near the size of gaps on the London underground!).
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