The outer ends of the trains were not fitted with fully automatic couplers, so that in an emergency, two trains could be coupled together mechanically, but not electrically. Once the 1959 stock was delivered, the couplings were modified so that the two builds could work together. The electro-pneumatic brake was of a similar design to the 1938 stock, but the motor generator set was rather different, as it needed to supply alternating current at 110 V and 850 Hz. The design was influenced by problems with the fluorescent lighting supply on the sub-surface R stock, which tended to generate a loud magnetic hum. The frequency was chosen to ensure that it could not interfere with lineside signalling.