Usually a combination of 2 or more locomotives usually on the head of a freight train (especially applies when there's more than 3). Most commonly used when something unusual or pleasing to the gunzel is running the train.
Example: Got a kickarse lashup on 9121 yesterday - XR553, G533, T399, T408 and X31 all powering through Meredith.
Road trains actually have at least 38 wheels (10 on truck itself, 4 on C trailer, 4 on B trailer and 6 on A trailer) . . . The Australian term for '18 wheeler' is semi or semi-trailer, as Australian trucks with just the truck and one trailer have 22 wheels (triple axles on the trailer).