Hi, smallest standard gauge steam locomotive I'm aware of is ex-Shropshire & Montgomeryshire Railway, UK, 2-2-2T, which weighed 5.5 tons and had 4"x9" cylinders. And yes, it could & did pull a coach. It still exists at the Colonel Stephens Museum. See http://www.hfstephens-museum.org.uk/locomotives/gazelle
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