A heavy rigid vehicle towing two trailers must be:
an A-train (a rigid vehicle connected to a semi–trailer that tows a full trailer), or
a B-train (a rigid vehicle attached to two semi-trailers connected at two points of articulation, where the forward distance of the longer trailer divided by the forward distance of the shorter trailer does not exceed 1.4, or
a heavy rigid vehicle followed by any combination of two trailers (not an A-train or a B-train), provided the trailer at the rear of the combination has a gross vehicle mass that isn’t over 3.5 tonnes, and the gross mass of the combination is less than 20 tonnes.
For any combination of a rigid vehicle towing two trailers, the maximum allowable length (without collapsible mirrors) is 20 metres.
The rear trailing unit distance means the distance between the rigid vehicle’s point of attachment on the first trailer and the rear of the combination, including its load. The maximum allowable is 14.5 metres, and this applies to all combinations of a rigid vehicle towing two trailers.
The coupling point distance for an A-train means the distance between the rear axis of the semi-trailer and the centre of the tow coupling connecting the full trailer to the semi-trailer. The maximum allowed is 30 percent of the forward distance of the semi-trailer.
The rearmost semi-trailer in a B-train must not be coupled further rearward than the rearmost axle of the lead semi-trailer.
The maximum forward distance of rigid vehicles and trailers is 8.5 metres. Note that usually only one vehicle in a three-vehicle combination will be able to have a maximum forward distance dimension without the maximum overall combination length (20 metres) being exceeded.
The rigid vehicle in an A-train must have at least two motor-driven axles in its rear axle set. A-trains and B-trains must not have retractable or rear steering axles.
The combined gross mass of an A-train must not exceed 39 tonnes. The combined gross mass of a B-train must not exceed 39 tonnes, unless it has at least two motor driven axles in the rear axle set of the rigid vehicle.
The combined gross mass of any other combination of a rigid vehicle and two trailers must be less than 20 tonnes.
