John Wallace sent the photo along with some additional information. Firstly, it wasn't a Carl Weber photo (I've updated the caption), but likely printed from a negative he purchased for his collection.
The photo itself was taken in Middletown on June 4, 1950. The occasion was a fantrip sponsored by the Boston Railroad Enthusiasts, using I-4 class Pacific #1380 - very rare power for this line. The route was especially interesting as it originated in Boston, took the Shore Line down to Providence, the Highland west to Plainfield, the Norwich & Worcester south to Groton, then back on the Shore Line to Old Saybrook, north on the Valley Line to Middletown & Hartford, then the Highland back through Willimantic, Putnam, and Blackstone back to Boston. The line to Willimantic would be severed by Hurricane Diana just 5 years after this photo was taken.
Sounds like a really amazing trip - what I would give to be able to take a trip like that behind an I-4 today!