I imagine a typical underlay to be like a giant parking garage, with the ceiling being the floor of the buildings. You enter the building via a ladder leading through the ceiling into the building above. On low gravity worlds there may not even be a ladder, just an hole, and you have to jump up into it. You still have to walk from the train to the entrance to the building, and I think this is where the equivalent to dumpsters would be as well. The plumbing and electrical lines for each building are exposed on the ceiling of the underlay. So there's enough room to walk around, but it's clearly not meant to loiter in. When I said vehicles and pedestrians don't occupy the same space I was thinking more of how we're taught to treat train tracks vs how we often treat roads. Around where I live people often walk down the side or even middle of the road, even when the speed limit is 30 mph and the traffic isn't light. Sidewalks also butt right up against the road.
It may not be a contiguous ceiling blocking all sunlight. There are small gaps between buildings, not large, but enough so you register the first floor as being above the surface. There are also larger gaps for inner-city parks and such. I imagine pedestrian infrastructure existing between the “first floors” of buildings, like pedways.