My mistake, the freight will be trucked from the origin ramp, not station.
The freight that can make service being trucked from the origin ramp will be trucked to the proper Ground terminal. Express retains the rest. For example, a resi that orginates in LA that is going to NYC.
They could also truck from LA to Memphis and fly to NYC, or fly to Memphis and drive to NYC.
When I sorted at UPS we had to learn all service levels for every zip code. Some zip codes only got ground while others could accept ground, 3 day, 2 day and air. Loaders also have to know this information but only for the trucks they load. Our only exception in sort was Early AM which were all separated even if local.
UPS flies planes out of my building with empty air cans every day because it's cheaper to drive that extra weight all or part of the way instead of flying it the total distance.