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Actually, the NYC subway used rotary converters, not MG sets, for 600VDC traction power. It came in (on the IRT) at 11kv 3 phase 25hz AC, which was dropped down to 400something (not 480), fed into a rotary converter, then off to the traction switchgear. Some IND substations used mercury arc tubes, and some substations used 60hz rotaries. AFAIK, the last 25hz unit was retired in 2000, at nearly 100 years of age (!). All of the mecrury arcs are gone - I think those were actually the first to go because of the hazzards involved with repair of them - they were not sealed units and did require periodic repair, plus they had a 24/7 pumpdown via pumps on them. FWIW, NJ transit used the same basic system on the Hoboken division until '83 when they went over to 27.5kv traction.
Rotaries requied a tender to start/stop them as needed - they'd take them off line outside the rush hour to save wear and tear.
Virtually all of the NYC subway today is solid state fed off of 3 phase 60hz power. The signals are still 25hz in places though. NJT uses 100hz signal power which is in fact generated by geniune MG sets. A pole pig sounds really weird at 100hz and sticks out like a sore thumb in the substation...
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