You have the Connecticut Avenue Line of the Washington Metro where it has to go real deep cause Connecticut Avenue is way over 100 feet above Rock Creek Park and Parkway. The tunnels are beneath the river and roadway beneath the high bridge. Nonetheless, it explains why the Cross Bronx tunnels under the Grand Concourse instead of a normal bridge over the highway.
174th and 175th Streets under Grand Councourse in the Bronx has a large arched bridge in both locations with the subway dropping from the ceiling in the middle.
Wow, I just pulled that up on StreetView... I never knew that was there. I guess I never thought about how deep the subway would have to be there.
It would also explain SEPTA's Broad Street Line under Broad Street where it crosses over the Roosevelt Expressway. I always had a suspicion that was the case from way back as a kid-why did they have a viaduct like that?http://g.co/maps/jc52u