To carry their heavy freight traffic, the railways of New Zealand maintain some 33,000 wagons with an aggregate capacity of about 400,000 tons. About half of these vehicles have been placed in service since 1945, and are therefore of modern design. The types of vehicles provided for freight transport give some indication of the classes of traffic carried by New Zealand Railways. Typical vehicles are open “highsiders” of 15 tons capacity, for coal, manure, and other bulk commodities; double-decked sheep wagons to carry 80 sheep; steel box wagons with unusually wide doors, designed to carry 14 tons of palletised freight; bogie-type box wagons for the conveyance of merchandise at express-train speeds; louvred box wagons for fish and fruit; and high-capacity, bogie-type insulated wagons for the transport of frozen meat from works to ports. Other specialised types include massive bogie wagons especially designed for the transport of pulp logs from the Kaingaroa Forest to the Kawerau pulp and paper mills; four-wheeled flat wagons for the conveyance of palletised timber; wagons with removable hoppers to facilitate the loading of colliers at West Coast ports; tank wagons for liquid hot bitumen and tallow; and pressurised tankers for bulk cement. Another development is the use of lightweight portable containers to facilitate door-to-door deliveries. These containers can be carried on railway wagons or motor trucks, and are designed to fit into the cargo hold of a Bristol Freighter aircraft, as used on the Railways Department's Rail-Air freight service over Cook Strait.