Here's all the details and a link to the module on the Henning's Trains website.
30400 Legacy Writable Program Module
The Legacy Writable Module is a 128KB Memory module utilizing an I2C interface to store data.
The module is a direct replacement for the programming and locomotive parameter module for the Lionel Legacy 990 and BASE3 command sets. It is used to enable program updates to the Lionel Legacy system command base and CAB2 remote as well as to create engine modules to store characteristics of one or all the locomotive programmed into the command system.
