There are over 40,000 KM of highway in Canada, designated to be worthy of carrying very large-load transportation vehicles on a regular basis across the Canadian territories. These heavy transportation units carry materials such as raw agricultural and industrial supplies, heavy machinery, finished consumer goods as well as dangerous goods across Canada in large quantities. The Canadian road transportation industry is heavily regulated in terms of safety but not in terms of other factors, which makes it a free-competitive market with a constant downward pressure on rates and an upwards trend in terms of the quality of service provided.
The following are some of the medium-heavy, heavy and very-heavy transportation equipment in use across Canada and the 42 states.

Medium Heavy, heavy and very heavy trucks in use for freight hauling in Canada

Refrigerator truck

This type is a medium heavy transportation unit used to carry refrigerated items (like consumer products such a meat, fish or ice cream) in bulk. This type also contains an in-built cooling apparatus (which tends to increase its weight and its cooling-power requirements- necessitating the need to carry very large quantities at once).

Log Carrier

A log carrier is an especially deigned flatbed trailer used to transport large, very long sections of very heavy constriction timber.

Heavy load Flatbed trucks

Flatbed trucks consist of a drivers cab with a central mounted engine and a hydraulic controlled, wheeled flat wheeled detachable platform at the back, called a flatbed trailer (or a semitrailer which is smaller) which can carry heavy or very heavy loads of a wide variety such as shipping containers, machinery and irregular shaped objects that require space
The medium heavy varieties are used with a full trailer (not semitrailer) and are used to carry machinery such as industrial devices (huge factory machine segments like giant boilers or generators) and other machines such as constructional diggers, cranes and backhoes. It is also the preferred method to load construction material into flatbed trucks using cranes (which cannot load enclosed containers and semi-trucks) in bulk - such material is normally; heavy pallet loads of bricks, concrete blocks, cinder blocks, bags of cement or plaster and other very heavy loads of this variety (like gravel).
Other types of construction and irregular material carried by heavy flatbeds are construction steel (rebar, which are very long in length and very heavy especially for the construction of tall buildings - necessitating the use of crane loading onto flatbeds), huge drainage pipes which are long in length and cannot be carried or even loaded onto semi trailers and lastly, long and heavy steel girders used in building skyscrapers. The military uses these types of transports to carry artillery pieces, tanks and parts of aircraft and ships.

Semitrailer Truck

These are the large types of hauling trucks and are used or specific types of goods such as containers and the type of items mentioned above. The standard requirements (department of transportation) for transportation on the national highway system limit these to 102 inches wide, 13.5 feet tall and with a maximum limit weight of 80,000Lbs gross, however, individual states can issue special permits for oversize and overweight transportation. Standard trailer are allowed up to 28.5 feet in length and LCV's (longer combination vehicles) are also allowed by many states (LCV regulations vary in Canada and from state to state in the US).
LCV types:

Triples: Three 28.5-foot (8.7m) trailers; maximum weight up to 129,000 Lbs.

Turnpike Doubles: Two 48-foot (14.6m) trailers; maximum weight up to 147,000 Lbs.

Rocky-Mountain Doubles: Include a combination containing one 40 to 53 foot trailer (maximum allowed is 48 feet) and one 28.5-foottrailer; maximum weight up to 129,000 Lbs.

In Canada, a Turnpike Double is two 53-foot trailer combinations and a Rocky-Mountain Double is a 50-foot trailer with a 24-foot trailer.

References:
Extensive research Wikipedia[dot]com
www[dot]fastfreightquotes[dot]com/freight-services/freightequipmenttypes.php
http://hubpages[dot]com/hub/flatbedfreight