Definition of "siderod"

I recall in particular the amazing speed of the siderods of our elegant little Caledonian 4-6-0 and the vivid contrast between its polished blue livery and the red-funnelled mail steamers and white-hulled yachts crowding Oban Bay; […] .

1951 October, R. S. McNaught, “Lines of Approach”, in Railway Magazine, page 705