Have noticed 10 and 12 M3 trains on the Harlem line but never more than 8 car M7 trains. Heard that the substations have to be upgraded. Is that the case?
correct the M-7's have much higher Horsepower than M-3 (1036 vs 660)
not sure how much was upgraded, but for example near Bronxville there was a trailer type emergency substation for years it would need total replacement.
north of Virginia Road sub stations as build were only for 6 car M-3 trains , later upgraded and voltage raised to be able to run 8 car trains.
below Woodlawn there was some spare capacity and some M-8 trains are run with 10-12 cars (same 1036 hp)