Originally Posted by
lincolnjkc
there were both architectural (grand gateway/something worthy of Saarinen's terminal) concerns but also passenger circulation considerations, e.g. the tunnels need to fall below the TSA checkpoints (and presumably baggage handling, etc. but I've never been behind the scenes at IAD) and also the pedestrian connector to A/B -- so you have to dig out a big hole anyway you can either fill it and make it feel like LaGuardia used to or celebrate it.
The Main Terminal AirTrain station sits just south of the original Main Terminal and was built as a cut-and-cover station, and there's nothing above it -- its roof (with its skylights) sits roughly at ground level, right under the moon buggy docking area. So I guess you could have built some very expensive space for... something... above it, or you can have a vaulted ceiling.