At North American Rayon in Elizabethton Tennessee, they ran a Porter fireless 0-6-0 around the plant from their steam boiler, as late as the early 1990s.
That might be the very last commercially run steam locomotive not used for tourists or a museum that I can think of. It never turned a wheel hauling people that I know of.
It ran interchange with ET&WNC 2-8-0s number 207 and 208 (today known as Southern RR #s 630 and 722), making this spot the last place in America where steam from two different owners worked in interchange, as late as 1967.
