At my second home, Seatac airport, there is a subway. Actually there are three subways. They are used to connect the central terminal to the north and south satellite terminals.
The line itself is fairly old, but they renovated it a couple years ago.
But it makes no sense to me.
To get from the N gates to the S Gates, you take the subway to the central terminal, switch to another subway car that takes you to the other end of the central terminal, they switch to another car to get to the S Gates.
All these lines run close together, and I imagine there must be common service tunnels under there.
Does anyone know why, when they renovated the entire system, they didn't just do away with the separate lines and make one loop? It might make a trip a little longer, but it would be much more convenient and simpler to get from one place to the next.
It must have made sense to someone.