My guess is at a minimum maintaining the bearings would be more difficult. A solid axel allows placing an external bearings between the truck and the axel. H

My guess is at a minimum maintaining the bearings would be more difficult. A solid axel allows placing an external bearings between the truck and the axel. Hub bearings are internal and service would require dealing with the nontrivial weight of a loose wheel. See https://en.m.wikipedia.org/wiki/List_of_railroad_truck_parts https://en.m.wikipedia.org/wiki/List_of_railroad_truck_parts