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As built the 159s had 4 step brakes, the 158s had three step. This meant there was an extra contact on the coupler. It was possible to couple them to another sprinter but only in a certain order (IIRC the sprinter had to lead the 159). This was reported in RAIL around the time the units were being built. They've since been modded for flexibility. *
Though it's possible that 4 step braking was proposed but then not fitted and RAIL got the wrong end of the stick.
I think that class 159s had a special arrangement for locking the front vestible out of use to passengers. The idea being that it was reserved for the use of the driver to go to signal post telephones on the busy approaches to Waterloo. Class 158s were not permitted in to Waterloo for some years because they lacked this arrangement,