There is a New Haven system map from 1922 here:
http://www.rootsweb.ancestry.com/~canma ... HH-RY.html
It's the sort of advertising map many railroads produced, with too many geometrical/geographical distortions to help with my question... but it does show the Meriden to Waterbury line as still officially open in 1922.
There is a 1946 U.S. Geodetic Survey map of the relevant area here:
http://historical.mytopo.com/getImage.a ... g&state=CT
(The same WWWebsite will also give you the 1955 USGS map.)
For the immediate area I was talking about, scroll to the lower right-hand corner. The center of the village of Cheshire (village green, etc) is about an inch from the bottom. West Main Street is the red street heading off roughly in the 10 o'clock direction from just left of the "C" in the name "Cheshire" marking the village. (The village is just one neighborhood in the town, and there is another "Cheshire" -- in larger type -- elsewhere on the map marking the town: ignore it and look for the smaller name an inch from the bottom.) It slopes fairly steeply down to the place where it crosses the Canal Line: I think it would be surprising to have a steam railroad going East-West at this point, but a trolley line, maybe! The map does not show roadbeds of abandoned railways.
CSOR2010: you have solved my problem! The roadbed I found was only a very short distance north of West Main Street, so it has to have been the trolley line: the prison (labeled a reformatory) is shown on the map just linked, and is some distance further north.