I had heard of 'transfer trailers' a for a long time before I figured out what they were. However if you have a need to move aggregate a long distance this is one of the solutions particularly if you need to dump it when you arive at the destination. There are several solutions to the problem. You can use a belly dump but this usually means that you have just a semi truck which cannot qualify for as much weight under western bridge formulas as a truck and trailer. You can use a Pup trailer behind a dump truck, but those have a problem if you have confined quarters at the destination (as does a belly dump) or you can use a transfer trailer like you see here.
When using one of these units, you load and travel to near the destination like a truck and trailer unit. When the vehicle arrives at a marshaling area, the trailer is disconnected, and the truck proceeds to the final destination and dumps the load off the truck like a conventional dump truck. Then the truck returns to the trailer, and backs all the way up to the trailer. Note how the trailer tongue is a special 'low rider' which will allow the truck to straddle the tongue laying on the ground. Note in the second photo how the truck tail gate folds up out of the way. The transfer box actually sits on rails and is then pulled inside the truck box. Once this is done the truck once again proceeds to the destination and dumps the second load. The truck then returns to the trailer--- ejects the transfer box back onto the trailer, and the trailer is rehitched to the truck and returns for another load.
Clearly this sort of an operation has a lot of overhead in terms of hitching, unhitching, transferring and ejecting, etc. however the method allows an axle configuration to maximize the gross vehicle weight, something that you simply cannot do otherwise except with a configuration that involves pulling two trailers, and double trailers are a problem all by themselves, particularly if the delivery point is off highway. This sort of a setup allows you to go the 'final mile' essentially as a traditional 10 yard dump truck, but allows you the luxury of only returning to the 'reload point' for the second load instead of returning all the way to the quarry.