There is a picture of the Sub Caverns during construction a page or two back - you see the man and ladder on the left, and a row of pillars on the right curving left into the background. One or more of those may have a Monorail pylon directly over it, depending on where that shot was taken.
If a Sub cavern roof pillar has a Monorail Pylon on the top of it, they just make the footing pad at the bottom bigger and put some extra steel rebar in the column to handle the added static load - doesn't necessarily need to be any bigger diameter than the ones nearby, just built a bit heavier to handle the weight.
The sub cavern roof will distribute most of the lateral stresses, they don't need to be transferred down to the footings.
--<< Bruce >>--