I am a retired archiect and have studied the rather simple section of the tunnel. It seems to me that there was a central walkway and two delivery compartments, one on each side. The right hand compartment seems to show clearly a tall rolling service "cart" with perhaps two levels, riding on tracks. This is not shown on the left side but the fact that there are structural metal wall supports shown on th left side may simply mean this was to illustrate that the concrete tunnel was reinforced with steel or iron. Probably both sides had a cart system. A round vent, pipe, or conduit seems to be resting on top of the cart compartments. The tunnel, given its length, must have been supplied with ventilation, electrical lighting, etc. and probably a telephone line to connect the two buildings. it is probable these conduits were for that purpose. Probably the carts were powered in some manner by electricity. Highly improbable that they were pushed or pulled by servants. It can be also surmised that there wee drains along the way in case of spillage from the carts or ground water entering the tunnel. There appear to be rather thin side panels on both sides of the tunnel walkway separating the railed carts from the walking area. These were probably designed to slide or swing open to allow access to the carts should there be a mechanical problem en route in the tunnel. Given there does not appear any way for carts to turned around at either end, I surmise carts simply went back and forth, wiith someone delegated at one end or the other or both, to telephone the opposite end and control direction.
