The PFM Sierra #38 is much too large to be the logging locomotive as used by the prototype, however there were plenty of 57" drivered 2-6-6-2 mallets used on other railroads. What prototypes can be modeled using the PFM Sierra as a starting point? I remember MR many years ago had a series on making it into a 4-6-6-2 cab forward look alike (that prototype had 63" drivers) and susequently there have been brass mdoels of the correct size for the SP AM-2 class engines.
I'll offer Northern Pacific #3100 (later used by Poulson, Rayonier) which had 55" drivers and was converted to an oil burner as a possibility, but some details like steam lines and piston valves are different.
Kenneth R. Clark
P.O. Box 127054
San Diego, CA 92112