The only 726 with double worm drive is the 1946 version. It has nickle rims, removable wheels, and originally came with a smoke bulb. The cab for that loco is unique.
In 1947 Lionel retooled the cab, and redesigned the drive train to drive a single axle. The rest of the wheels are driven off the side rods. There are some minor variations, but in general, all Berkshires from 1947 on share the same drive mechanism.
Lionel switched the number of the Berkshire from 726 to 736 when they added magnetraction in 1950.
In 1952 there was a shortage of magnets, so Lionel make the berks without magnetraction, and used the number 726. Sometimes the cabs are stamped "RR", other times not.
The 1952 726RR did not have magnatraction. The best place to find a shell for your locomotive is eBay. Patience is the key. They show up regularly and sell for approx. $40 -$50.
Jim
Modeling the Baltimore waterfront in HO scale
Don U. TCA 73-5735
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