I was just looking through Chris Baer's chronology on the PRRT&HS website, and it states that after Jan. 16, 1933, when through AC operation began between New York and Philadelphia, the third rail was removed west of the Bergen tunnel portal. This would mean that any trains that still changed engines at Manhattan Transfer (and not all the clockers were electrically powered until Feb. 1) would have had to use AC power between Penn Station and Manhattan Transfer. Apparently this would also have applied to South Amboy trains until about Sept. 9, 1935, when electrification was extended from Rahway to South Amboy Jct. (Dec. 12 to South Amboy station), and MU service began. After that, the only reason even to stop at Manhattan Transfer was, as BaltOhio indicated above, to permit passengers to transfer to and from H&M trains until H&M began using the new Newark station in 1937, and no need to change power on either through trains or South Amboy-NY&LB. It does look as though AC power may have been a common sight at engine changes there from 1933 to 1935.