I'll take a stab at this. I think you may be refering to a Roberts Track Laying Machine. Steam powered and equipped with booms. Ties and rail were fed ahead of the machine from flats loaded with same at the rear. All of this was pushed by a locomotive or two depending on the grade. The Moffat was using one of these machines about the same time as the Farmington Branch. The Moffat machine is the only photo I have seen of the machine. I don't know if the machine was self contained (it's own steam supply) or if the steam came from the locomotives.