In Chicago on very rare occasions when the train is running late they will have an impromptu "express" train where they skip the next 4 or 5 stops. They announce it so you know not to get on the train if your stop is being skipped.

It seems like such a huge deal for them to do this - it's very rare. It must be a giant pain, even though it makes a lot of sense. Not only does it get the train back on schedule, but it reduces the riders on that train, which further speeds it up. If there was an easy, intelligent way to allow the bus to go into "express" mode and alert the riders then the busses could avoid getting bunched up.