The term distinguishes electric rapid transit with single-unit or short-consist latter-day streetcars (although the transit professionals will tell you how they differ, thirty ways from Sunday) often sharing the roads with motor vehicles, as opposed to heavy rail transit with multiple-unit cars using dedicated rights of way, often elevated, in subways, or in the median of an expressway.
But the light rail vehicle (and its contemporary streetcar kin) is heavier than the trolleys it replaces. Thus, if Boston's transit authority replace the PCC units running on the Ashmont High Speed Line, they might have to buy new track structure as well. "If modern light rail cars were chosen, as elsewhere in city, the line will require rebuilding as these are much heavier than the PCC cars." That is, if Boston buy cars similar to the Green Line cars that run in trains downtown. Perhaps they could look into something lighter.
The problem, though, is that you can't go over to Home Depot and buy PCC parts any more. "They were built in 1945/46 and are the oldest passenger vehicles in continuous regular public transport service in the USA." The Kenosha PCC service uses former Toronto cars, and the service stands down during the deep mid-winter.
Arguably, though, San Francisco's cable cars are older, although there is probably an element of grandfather's axe in any statement about their age. The cars are listed. Also, and a lot closer to Cold Spring Shops headquarters than Boston, there is the Fenelon Place Elevator in Dubuque, and those are clearly passenger vehicles running on those rails, although they, too, stand down for the winter.