and Heavy tanks were used in the early stages of the cold war, but have gradually been phased out by the multi-role Main Battle Tank. Heavy tanks grew to the point of being logistically problematic, such as the Conqueror and IS-3, while the MBT became capable of filling their battlefield role in a comparatively Medium form-factor. In most cases, the Heavy tanks grew so large that they could not be transported by rail, and could not be supported by common bridges. Role classifications Many types are also described by their tactical role, which depends on contemporary military doctrine. For instance, 'infantry' and 'cruiser' tanks are British classifications of the 1930s and '40s; 'infantry', 'fast', and 'breakthrough' are Soviet types of the same time period. British and Soviet tacticians up to the time of the Second World War classified tanks into three major roles: infantry, light, and cavalry. Infantry tanks supported infantry units, to integrally support dismounted infantry actions. Light tanks performed the traditional cavalry role of scouting and screening. Cavalry or "cruiser" tank units were meant to exploit breakthroughs and fight other armoured formations. As role based classifications evolved, the role of light tanks was overtaken by other vehicles, such as carriers and scout cars. The infantry and cruiser tank roles were combined in British use late in the war to form the Universal tank concept. This was made possible as increased engine power provided the capability to sufficiently armour a cruiser tank, the Centurion, to undertake both roles. Centurion entered service just as the war came to an end. Post-war, tanks were similarly made capable of fulfilling multiple roles on the battlefield, resulting in the designation Main Battle Tank. World War I Initially on the very first tanks, two types with two roles were provided: the 'males', armed with two naval 6 pounder (57 mm) guns and machine guns, and 'females', armed with only machine guns that supported the 'males'. Later tanks armed with a single gun in one side sponson and machine guns on the other were named "hermaphrodites". World War II Tank models were developed before and during World War II according to different philosophies, with different combinations of armour, mobility, and armament. Each major nation developed its own doctrine of tank use, and therefore different tank models to suit. New doctrines explored the role of the tank as a fast-striking unit. Tank doctrine in the UK