The pictures I've seen of the lower deck (in a couple of Key System books) show an off-balance setup. Tracks are on one side (the north?). 3-lane road is on the other side. 3-lane because that's how passing was done Back Then - suicide lane. Tracks were *not* in the middle of the road.

I was barely noticing things when the tracks were removed and the bridge converted to 5 lanes each way - I'm thinking 1958-9-60? Do remember the "Hump" when the deck was being lowered through the tunnel so there would be clearance on the upper deck for trucks - given the panic caused by a simple slow-order S-curve in the East Span construction, I could just imagine what the lawyers now (and the zoned-out drivers) would do with a "hump" like we had back then. Lower deck of the western span still has the excess height needed for the overhead for the trains.