Depends. I use an iron for boiler bands, a torch for washouts and lag clamps, and a combination for running boards.
On sheet metal (cabs, tenders, etc) I use an 80 Watt iron, shiny, with good flux. Never do a seam on sheet metal with a torch. I do use a torch to attach handrails on tender and cab bodies, but the heat never gets close to the wire.
