Is this what you are referring to?
From Wiki :-
Although all auxiliary equipment such as lighting, air conditioning and air compressors was powered by motor alternators driven from the 25 kV overhead line, it was recognised that if there were a power failure, conditions in the passenger vehicles would quickly become unbearable and even unsafe. Each driving van trailer i.e. the leading and trailing vehicles, was equipped with a diesel-alternator generator capable of supplying the minimum requirement of auxiliary power. The diesel-alternators were started using air motors powered from the train's air system, since the APT carried few batteries.