Andy;
The 12073 is a short track section which includes an electrically isolated center rail option (there is a removable jumper wire on the underside). Not sure about the 12000X, but it may refer to the small sections that come with Fastrack O72 switches. The 12073 are sold separately, not sure about the switch pieces, but both are the same length, however, the pieces from the switches may only have the sloped roadbed on one side or on neither side. Hope this helps.
