Curious Escalator Math

This puzzler came from Irving Biggio. Here it is:

He writes, "Unlike many of your puzzlers, this one is actually about transportation. In our fair city of London, England-- not far from the real Cambridge—many of the Underground, or subway, stations have three escalators.

It's always the case that two of them are up escalators, and one is a down escalator. It doesn't matter which way the rush hour crowds are moving, or if more people are arriving or leaving, or the time of day: There are always two that go up, and one that goes down. The same is true for underground stations out in the suburbs."

Why is this?

Now, Irving's been kind enough to provide some wrong answers to explain this phenomenon.

For example, it's easier to go down than to go up. Or, people going down can also take the stairs. But, he adds parenthetically, most of the Underground stations don't have stairs.

He also provides a hint: When the station with the escalators is elevated above ground, the opposite situation holds. That is, there are two escalators going down, and one going up.

Think you know why the Brits have designed their subway this way?

Think you know? Drop Ray a note!

[ Car Talk Puzzler ]