It is also unclear to what extent the present-day, twin-axled, sliding wall wagons were standardised. Class G – Ordinary covered wagons The UIC's ordinary covered wagon class has rigid, fixed walls with sliding doors on each side. The upper third of the side walls has closable openings of various types. These may be designed as ventilation openings, loading hatches or combined ventilation and loading hatches. Today, Class G wagons have been largely superseded by other classes. The transportation of part-load goods that are susceptible to the weather in boxes, sacks and barrels is the main function of G class wagons. One disadvantage they have is that the single side door does not facilitate rapid loading and unloading of large unit loads. The demand for G wagons fell with the increasing use of intermodal freight transport. Today the majority of part-load goods are moved in ISO containers, for which special flat wagons are available. Where there is a requirement for the direct loading of part-load goods, wagons with sliding sides are preferred (see below), because they enable a faster transfer of pallets. G wagons were also frequently filled with bulk materials that were vulnerable to the weather. Special wall attachments were developed, especially for the transportation of grain, with which the side doors could be closed. Today there are self-discharging wagons with opening roofs available for this type of freight that are quicker to load and unload. G wagons were often attached to passenger trains in order to transport express goods and post. Special variants were sometimes used that, for example, were fitted with suitable braking equipment or heating pipes. G wagons were also used frequently for the transportation of cattle. This required good ventilation, which is why wagons were initially fitted with four, later eight, ventilation openings. Class H – Special covered wagons Older wagons grouped into class H These wagons were based on the standard covered wagon but developed for special roles and were always built in smaller numbers. All types were still around for the introduction of the UIC classification in the 1960s, but were classed as special wagons due to certain special features and retired almost completely by the end of the 20th century. Livestock wagons German wagon classes: to 1921 Ve…, from 1922 V… UIC classification: H…e… Whilst ordinary open or covered goods wagons were used for the transportation of cattle and other large animals, special