B&M trash trains?

Did they run them pre Guilford or was that not yet "a thing" yet?

It wasn't a thing yet, AFAIK.

Could scrap trains technically be classified as "trash"?

I don't think so. Scrap has market value and is being shipped somewhere to be melted down for reuse; trash is just trash.

When the B&M existed, you could still bury your trash in a town landfill, as opposed to today, where there are fewer places to bury it.

The B & M was poised to run trash trains about 1970. An outfit named Reclamation Systems built a facility in East Cambridge below the Prison Point Bridge. Reclamation accepted trash and baled it. It was to be transported to, Carver, I think. However, the company and related businesses could not get permits to dump where they wanted, so they had to settle for trucking out the trash. Mysteriously, Reclamation had a bad fire which destroyed its facility and took the Prison Point Bridge with it. The whole deal literally stunk as the head of Reclamation also had ties to the B & M.

Correction to previous post. Reclamation Systems could not get a location on the B & M to take the trash, so whatever trash they had compacted was trucked to Carver.