I believe the elevator you reference is in T-4, the American Airlines terminal. If you are going from T-6, then it would be at the opposite end of the tunnels. I've only ever used the escalator/stairs to descend from T-6 into the tunnels. At T-4, the elevator is next to the stairs/escalator if you are on the departure level, on the tunnel level the elevator is "behind" the stairs/escalator.

The carts that go between the tunnels are not "regularly scheduled" - you just have to wait for one to show up.