I probably should just sit on my hands and stand aside for those who had more first-hand experience of what happened before I was in S scale, but my study of the past literature (and owning a Rex Suburban myself) suggests that what you have is the re-run of the Rex by Jack Sudimak as Putt Trains, indicated by the Mashima motor and the nylon worm and gear.? I had one of the re-issued 2-6-0's with those modifications, as distinct from the brass worm, fiber gear, and PIttman motor in my other Rex locomotives.??

As to the trailing truck, I had not known that either Rex nor Culp ever did an Andrews truck; so far as I've been able to learn, the first Andrews was the Northeastern (later Kinsman), and yours doesn't look quite like that.? The Rex trailing truck usually had working leaf springs, but I suppose some were made with coil.