These mammoth copper-strand cables indeed were heavy and thick, barely fitting into the insulators' side grooves. They were cloth covered with (I believe) a layer of rubber beneath, next to the core strands.

Their bare copper tie wire (solid, hard-drawn) was almost as thick as a dime and provided a rather tight fit!

A couple linemen I've known that have worked on Boston's transit-line DC cables stated that their size commonly was 1-million to sometimes 1.5 million AWG.

Click "Next" for an overview of the station wall where this closeup appears.