The Boston and Maine in pre-MBTA subsidy days (1965 and earlier) ran a few trains with 7-8 car consists. There's a B & M Employee Magazine article about 1956 that trumpets the longest assigned train at the time was a 7-car Rockport job.
In MBTA subsidy days RDCs from outlying points were brought into Boston Saturday morning, serviced, swapped, etc. and returned Sunday nights. The Ipswich set was up around 8-9 cars. These were also scheduled, passenger carrying trains.
After the most recent big Chelsea fire, Oct., 1973, all of the Rockport and Ipswich cars were coupled together into one mammoth, 17 cars by one account, train and run into Boston via the Saugus Branch.