train in opposite directions. Consequently, the bogies on the 'D' end motor car are installed the other way around from those on the 'A' end causing the two motor cars to move the train in the same direction. The 'A' and 'D' designation is from the designation of the axles on the two bogies. The 'A' end car has the 'A' axle at the driving cab end of the car with the 'B' axle on the opposite end of the same bogie. The 'C' axle on the second bogie is nearest the driving cab with the 'D' axle closest to the coupled trailer car. The sequence is reversed on the 'D' end motor car. This is also why the alternate car is not designated the logically more obvious 'B' end. Another mechanical difference between the two types of car is that the 'D' end car has a spring-loaded buffer at the driving cab end whereas the 'A' end car has a solid buffer. These differences mean that when coupling two train sets together, the 'A' end of one set must be coupled to the 'D' end of the other. Although train sets are not normally split in day to day usage, they do have to be split when undergoing major maintenance tasks, such as when the bodies are lifted off of the bogies. This is not a problem on most lines, where all the train sets start and remain the same way around. However, on the Northern line the existence of the Kennington loop, just south of Kennington station, which allows Charing Cross branch trains to depart from its southbound platform and arrive at its northbound platform reverses the complete train once every journey. This means that, on this line, on average, half the trains will be facing in the opposite direction to the remainder. Only sets facing the same way can be coupled together. Traction control The 1938 design stock features the (by this time) standard series/parallel traction control using bridge transition. The control system has an unusual feature. Ordinarily, the driver moves the master controller from 'off' up to 'full series' and then further to the 'full parallel' position when the train will be under full power. The unusual feature is that once the driver has moved the controller to 'full parallel' and the train has accelerated to the point where the switchgear is actually in full parallel,