Washington Dulles built an expensive train to carry passengers between terminals which started operations in 2010. But they don’t really do that, at least they don’t do it for all passengers and they don’t take passengers of the airport’s major airline anywhere near their gates.
Instead they also have a system of “mobile lounges” or moon buggies that carry passengers slowly across active taxiways to their terminals. Many United passengers still have to use them, for instance.