Based on the description in that link, Phase 3b seems like the best match, judging by the photos of 52 and 77 I've looked at. Definitely not Phase 4, and probably not anything earlier than Phase 3a.
MEC407
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Pan Am Railways — Boston & Maine/Maine Central — Delaware & Hudson
Central Maine & Quebec/Montreal, Maine & Atlantic/Bangor & Aroostook
Providence & Worcester — New England — GE Locomotives