Since it is a new program, the presence of an engineer shouldn't come as a surprise.

And FWIW, I do this exact same sort of work - driving trucks transporting US mail intercity and interstate. I physically handle the mail about 10% of the time. All of it is loaded into transport units that are on wheels or palletized.

One of the most common pieces of equipment looks like this, called a "GPC" or "General Purpose Container" ;


Note the yellow device with the round hole at the top. This is a hitch so several can be hooked together and pulled with an electric Tug. The opposite end has a small length of pipe of slightly smaller diameter that the round hole fits onto.

The Tug looks like this, though this is a brand new unit and most in current use are much older designs, but they look very similar;




Others include "BMC" (Bulk Mail Container), Wire baskets (Simply called "Wires" ) and Canvas Hampers.
Here's 3 of the primary and most common containers in one photo;



"CIN BMC" would stand for "Cincinnati Bulk Mail Center". If you look carefully at that aluminum container, you can see a handle that has a mechanism hanging straight down from the center of it, with a pin visible at the bottom. That is a locator pin which can be used in the Distribution and Sorting centers that have in-floor conveyors, basically a chain that runs just under the floor level that is accessible via a channel. The pin is dropped through the channel and a hitch will grab the pin and pull the container along to the various sorting stations it might need to go to.

The mail hauled in the trucks mentioned in the OP travel from one sorting center to another, and is then sorted for street delivery. More often than not, the driver either places the trailer against a doc and drops the trailer or places it in a numbered parking space and a yard jockey truck comes along and places it on a dock.