Not a duff loco, just the mechanics of a long coupled wheelbase having to be got round a curve. The diesel chassis have considerable flexibility with a pair of independent short wheelbase bogies with centre gear coupled wheels. On the steamers the powered wheelbase is usually longer, and the coupling rods create binding as the wheels move laterally in the chassis for the curve: every additional pair of flanged wheels makes the effect worse, so your 9F will be worst of the lot. If you have not done so already, check that all the 9F's wheels have unconstrained sideplay and that all the coupling rod pins are lubricated, as this will help.
It may interest you to know that I loco motorised the original tender drive Triang Hornby 9F when it first came out, and that ran on a layout with mostly set track curves. Despite attending to the lubrication this loco wore through the coupling pin holes in the rods after a few years operation, and had to have them rebushed.
It may interest you to know that I loco motorised the original tender drive Triang Hornby 9F when it first came out, and that ran on a layout with mostly set track curves. Despite attending to the lubrication this loco wore through the coupling pin holes in the rods after a few years operation, and had to have them rebushed.