Being involved in transformer repair and rebuilding, I am often asked "What is a Type R solid core?"
Lionel introduced the ZW in 1948, as a 250 watt transformer. In 1950, Lionel was given permission by UL to upgrade the unit to 275 watts. In 1957, Lionel introduced the Type R, still at 275 watts and furnished it until 1966 when they stopped manufacturing ZW's.
Now the basic difference. ZW's manufactured prior to 1957 had a core made of loose laminations. The laminates were "C" shaped and after the primary and secondary coils were wound the laminations were inserted into the coils with the back of each "C" on alternate sides of the coils. This was and still is a very common way of manufacturing magnetic cores.
When assembled, the laminates fit snugly against the side of the coils but with age and heating and cooling the coils would sometime expand and the laminates would loosen up. The result of the loose laminates is the creation of a 60 cycle hum. Normally you can hardly hear it, but occasionally it can be loud enough to annoy. Electrically, a slight hum does not create a problem.
In 1957, to resolve this problem, Lionel introduced the Type R solid coil design in both the KW and ZW transformers. The large component of the core was "C" shaped and the laminates were epoxied together and the small component was straight, made up of epoxied laminates and after the "C" portion was inserted into the coils, the straight piece was epoxied across the opening of the "C" to form the loop necessary for a transformer to work. Hence, the laminates being epoxied together they never loosened up so no 60 cycle hum.
One caveat. Sometimes someone will have a Type R that will start to hum and vibrate so badly that it will try to dance across a layout. What has happened is the epoxy holding the straight piece to the opening in the "C" has loosened up creating a minute gap and that really causes a 60 cycle hum/vibration. This is an easy situation to rectify, but does take a bit of time.
Lionel never indicated the Type R on the nameplate of either the KW or ZW. Sometimes it was stamped on the bottom plate but bottom plates get changed out and/or repainted so the only way to truly identify a Type R is to look at the coil and it is very obvious. See attached pictures.
If you have anymore questions, let me know.
Jim
