Most freight and goods for American towns in the 1920s came by train. In the mid-1920s, some 75%-80% of intercity freight moved by rail. Freight handling in the 1920s, whether at truck terminals or at rail transfer points, involved a great deal of hard, manual labor.
Rail classification yards that sorted rail cars into trains going to particular destinations were just one part of the distribution story. Railroad 'switching crews' took care of sorting rail freight cars; small locomotives chugged around the yard, moving strings of cars from inbound storage track to 'make up' tracks, on which cars were grouped by route or destination. Switching conductors were the bosses: they directed the movement of the 'cuts' (groups) of freight cars by the locomotive and directed their assigned brakemen as they manually uncoupled and coupled cars. The switching-locomotive engineer followed the conductor's instruction as to any and all movement of engine and cars; the fireman stoked the firebox.
But a great bulk of freight traveled 'LCL' - i.e., in less-than-carload lots: dry goods and canned goods heading from manufacturers and distributors to store owners and managers, farmers' seed and tools being shipped on order, mechanical parts going to factories for repair of the factories' machines, bulky Sears or Montgomery Ward orders, etc., etc.
That's where 'transfer sheds' came in. Just as a large classification yard was a hub for rail cars, a transfer shed was a hub for LCL freight. The Spencer Transfer sheds were the main hub for most less-than-carload shipments for a large part of the central Piedmont region served by the Southern Railway. Box cars coming on the mainline from far away had to be parked at the sheds and their contents unloaded and re-sorted by hand into other box cars that were to go out on local freight trains or on secondary and branch lines to individual towns and cities.
It was an operation just like the 'break-bulk' unloading/loading of a traditional cargo ship. Instead of cargo hooks on the docks, men in the transfer shed used hand-trucks to move crates, bales, sacks, and barrels out of one car and over to another.