I believe Lockheed offered some built-in high-pressure air bottles for combuster type starts. Not used on EAL Electras. Some DC-8 had a cobuster on nbr. 3 engine. 3000-psi air was stored in the MLG oleo legs in a chamber above the oleo portion. There was also a small tank of 60/40 alcohol-water that was injected, mostly to improve mass flow I think. EAL also modified on Electra by installing an APU in the aft baggage area. It was gone by the time I arrived in 1964 as it was too noisy (or so I was told). That aircraft had no aft LH fuselage pneumatic ground connection to the confusion of ground crews.
Little known fact, first few EAL Electras had provision for a speed brake on fuselage belly midships. I opened a large screw-installed belly plate for a phase-check inspection and there was a cavity under it with hinges and mounts for a hydraulic actuator and capped hydraulic fittings. Of course with those huge flat face of the props there was no need for speed brakes. Later aircraft had skin with no sign of this.