Sac light rail has good sized rail - probably over 100#. But there seems to be a slightly wider spacing of (concrete) ties than on a freight line.

Let's face it - light rail cars aren't really light (ask those hit by one - if they survived), but they are generally lighter than heavy rail like BART cars. But they operate relatively often (about every 5-10 minutes most of the day in downtown Sac where all the lines come together). Plus, for track in the streets, you have to allow for impact of road traffic too. Even back in the 1960s-70s, when Muni was being rebuilt by BART, quite heavy rail was used.

I suspect the frequency of traffic, along with the relatively strong accel/decel, makes up for the weight of the cars in figuring rail weight needs. As far as the name is concerned - I'd blame the Feds in the late 1960s when they wanted something different for their DuWag-clones built by former aircraft makers; "streetcar" was just so 40s ...