Re: Small steam locomotives

Greetings:
Since you didn't specify country of manufacture, I think that Richard Farmer's GWEN 18" gauge, built by Hunslet in 1920 for a company in England weighing in at 10,000 lbs. (5 tons) is the smallest complete one in the US. It was brought to the US in 1958? (who knows the correct date?) by Billy Jones. Richard has done a wonderful job restoring it!
As for US built, the Risdon Iron & Locomotive Works c/n 9281, built in 3/1893, a geared 0-4-0T, long abandoned at Sheep Creek, Alaska, the remains of which (wheels, frame, cylinders and machinery) were recently brought to Juneau for display at the A.J.Gastineau Mill Tour. When complete with boiler, cab and tank, it weighed 6,000 lbs. (3 tons).
As for a US built, conventional rod locomotive, I think that H.K.Porter c/n 814, an 0-4-0T, built 3/1887 for Arizona Copper Co. "FORMAN" #4, on display at the (closed, but the locomotive is outside and can be seen through the fence) Arizona Museum in Phoenix, AZ is the smallest. I have it's weight at 17,000 lbs. (8-1/2 tons).
J.David