I used to use the station they trialed this in at rush hour frequently, and over the course of an hour I could believe 3,000 people ended up queueing for escalator space, which is compounded because people are trying to cross the standing queue to get to the right side and walk up. You've also got the flow of traffic from the down escalator doing the same thing.

It sounds crazy, but Holborn really does have vast amounts of traffic in the morning and evening, frequently enough that they'd have to shut the gates until the platforms had cleared enough to fit more people in.