I got to tour the remains of the loop under South Station in 1985 (as the major renovation was just starting). Part of the loop became a bowling alley and you could see some of its remains. Another part right under the center of the head house was used as an electric transformer vault. What was truly amazing was part the loop closest to Atlantic Ave. You could plainly see where the track bed once was as well as platform curbing.

Tom Coughlin
Stow, MA

Did the Union Freight RR, which directly connected between the yards near North and South Stations, ever have passenger trains on it?

Hi Jim
I don't think they ever had any type of passenger service, on Union Freight Railroad, as regular service. But how ever I did find One trip please read below.

I did find one trip that was sponsored by the Mass Bay Railroad Enthusiasts the trip took place June 16th 1940.
The route was the following Atlantic Ave ( Northern Ave) to New Haven connection at (A-street yard) on Atlantic Ave. Boston & Albany connection near South Station,- Atlantic Ave. then to Boston & Maine connection at yard 2, and return to engine house.
Equipment One flatcar with Railing
Motive Power Union Freight Climax build 1923.
This information is from the Mass Bay RRE newsletter Callboy 75 years of Rare Mileage trips Part 1.

Boston's Atlantic Avenue Elevated handled regular passenger service in that market. There were various service patterns including a loop using the Washington St. Subway (later the T's Orange Line) in one direction and the Atlantic Ave. El. in the other.

The Atlantic Ave. El. was damaged in the Boston Molasses Disaster in 1919 and was closed in 1938 and dismantled in 1942 for the War Effort.

Phil Mulligan

While performing the safety inspection of the new electrification system, I twice walked every inch of the line from New Haven to Boston including all of the trackage at South Station and don't recall seeing any remnants of the track under South Station. Of course since I'd never heard of it I wasn't looking for it anyway.
Ned