Doubles refers to a setup where one tractor hauls two trailers hitched together — often two shorter “pup” trailers. Running doubles lets a carrier move more freight per driver, which is why it’s common in less-than-truckload and parcel networks.
Pulling doubles takes extra skill and, in the US, a specific endorsement on the driver’s commercial license. The combination is longer and handles differently, so not every driver or route is suited to it.