GTW did roster 50-foot cars with end doors. Some lasted long enough to be sold off to other railroads (I remember one that ended up on the Michigan Northern Ry post-1976 and I am certain that the end doors were welded shut by that time).
Wabash also had some although it is uncertain that any would have wandered into New England.
As to the Ann Arbor (and I know more about their freight car fleet than any sane person ought to) it rostered NO 50-foot boxcars. There were 1920's-built single sheath cars including the one modeled by Speedwitch. These were neat cars with taller-than-usual Hutchins ends and were built as door-and-a-half cars with many of them converted to single door cars. Rob Adams wrote a comprehensive history of them for Ted Culotta's Prototype Railroad Modeling, Vol 2 and there is a picture of one of them in Maine on the BAR loaded with pulpwood. Some had racks for battery loading as the Globe-Union Company had a battery plant in Owosso, Michigan. Interesting operation that received zinc in Canadian Pacific boxcars and shipped batteries all over the place but, again, doubtful if any made it to New England. It wasn't until the DT&I assumed control of the AA (straying over the time line but in the interest of a complete answer) that there were any 50-foot cars with AA reporting marks. Small groups of DT&I cars were given "A.A." reporting marks and AA compass heralds to make it easier for assigned cars to be routed back to their shippers. There were both ACF and P-S built cars but none with end doors.
