I have several of each of these, and I use them much more often that I would have expected.
They’re simple weights. Cut with nice square corners, so they’re more than just a thing to hold something else down. I can use them to square up corners when I’m gluing them. They’re used to brace things against. They’ll hold the other end of track down as I spike it. They hold wires on the top side as I work with them belowdecks.
I bought some brass bar stock many years ago (about 1″x2″ x 5 feet or so), and had a friend cut off several pieces about 6″-8″ long. I still have one piece about 2′ long – not something I want to drop on my toes.
The larger one shown is a steel block, of which I purchased 3 or 4 at some modeler’s garage sale (alas, I don’t recall whose).
These aren’t precisely measured machinist’s blocks — the dimensions aren’t any nice even size. I don’t know they’re weight, but I do know the brass one are NOT sized to anything other than “this looks good”.
They’re both heavy, as a weight should be. I’d guess the steel weight at 5-8 pounds, the brass ones at 3-4 lbs. maybe.