Steve,
The rail kit is an internal part of the transmission. This is the brains of the shifting and making sure the transmission does not go into two gears at once. Most people do not recognize the wear or how important it is to the overall performance and life of the transmission. There were 6 different rail kits made through the years.
Rail Kit Identification
The rail kit consists of #21, 22, 23, 24, 25 and 27 in the following
Exploded View of a Toploader