I hear you academically but on the anecdotal level there is a residual sentiment that heavier gives better traction.

There is a story coming up here and it is kind of legend amongst Muni traction fans. I don't know if you ever met Charlie Smallwood - he told me the story of the 0131 and how it came to be. In 1957 Muni using a lease-purchase scheme to bypass the City charter obtained a large number of PCC cars from St. Louis, their 1700 class which became Muni 1100 class. At the time a gentlemen from St. Louis transit came out to SF and they had a meeting of muni management where questions came up about the new-to-Muni cars. Charlie's question was what did they do in ST. Louis when a PCC broke down on the line. The gentleman from St. Louis said we have one of our old heavy standard cars come out and drag the PCC back to the carhouse. IIRC Muni manager Charlie Miller responded make it so. Charlie had already stashed the 130 in the back of Geneva Carhouse for just such an opening. You used to enjoy the 0131 out on the street with a hapless PCC in tow back to Geneva in the sixties and seventies.