I was wondering about NH's commuter operations out of Boston. What locomotives were generally used by commuter trains in and out of Boston prior to the T's takeover? I'm curious about both the steam and diesel era.
In the period I'm personally familiar with in the 1960s, the New Haven's Boston commuter trains that were assigned standard coaches were almost exclusively powered by the NHs dual service GP9s, with RS3 526 (the last RS3 with an active steam generator) often assigned to Boston as well. Seeing other locomotive types was rare by the mid-60s. RDCs from the NH's big fleet were even more common and covered more runs than standard equipment, including most off-peak and weekend trains. Penn Central kept the ex-NH GP9s and added a few E8s ,as well as (briefly, in the early 70s) a couple ex-NYC RS3s than ran mostly on the Needham branch rush hour trains. RDCs became less common in the PC years. The MBTA purchased an assortment of GP9s, E8s, and RDCs from PC when they assumed equipment ownership.
If you're seriously interested in the subject you should track down a copy of the Boston Street Railway Associations excellent 1985 book Boston's Commuter Rail: The First 150 Years by Thomas J. Humphrey and Norton D. Clark.