Cab Signals on the Boston-New Haven

In the MBTA forum, I recently learned that there have been cab signals on Mainline/Shoreline as early as the New Haven days. When exactly were cab signals established between Boston and New Haven?

Cab signal territory on the Shore Line was from a point just east of Shore Line Junction to Cranston and from Boston Switch to SS-185 which was just a bit west of Back Bay Station and was later called "Chick". Cab signal territory also included all sections of additional track whether it was main track or siding as long as it was signaled track. There was much of that too in New Haven days. The Springfield Line also was cab signal territory although the system and the indications were much different between Air Line Junction and a point south of SS-274 (Springfield).
Noel Weaver

any particular reason for the gap in cab signals between CRANSTON and BOSTON SWITCH

Probably due to the cost and complexity of signaling what in those days was a very busy terminal area with lots of switches, sidings, and extra main tracks. Plus, much of the territory was speed restricted. It's the same reason why at the New Haven and Boston ends the cab signaling didn't include the stations or the immediate terminal district.

Thanks Tom for the answers, and that makes a lot of sense. Were there any other Interlockings located between CRANSTON and BOSTON SWITCH that are not there today?