Dave Wardale's response: Yes. Thermal losses from steam pipes are very small whatever the pipe size due to small temperature differences (zero for the main pipe and at most 100oC in the smokebox). On the 5AT, once the steam pipes leave the smokebox, they enlarge considerably to form part of the steam chest volume, which should equal the piston swept-volume (not so easy to achieve). Due to the "high saddle" design there is no part of the steam pipes exposed to the air - they are either within the smokebox or within the (heavily lagged) cylinders. The pressure drop through the pipe is influenced by the pipe-length/equivalent-diameter ratio, as well as by bends. The full equation for pipe friction flow in a most useful form is given in FDC 11.3 item . Minimizing this pressure drop through the live steam piping is of the utmost importance (= internal streamlining - Chapelon!).