The middle version from 1949 is probably the best of them.
The later version (1950 onward) with the 2-6-4 arrangement had a sheet metal trailing truck, and the earliest version (1948) had a smoke unit hot wire underneath that often came in contact with the ground rails and shorted the engine out.
The middle version (1949) had the wire problem fixed, while still having the die cast two wheel trailing truck.
