Well, actually the law itself hasn't changed, just the regulations issued by the FRA under that [enabling] law. Regulations have the force of law, and this situation leads to a lot of confusion and irritation, particularly with the small-government types.

The difference between a law and a regulation is that a law can be changed only by the enactment of another law (Congress, State Legislatures, depending on whose jurisdiction is involved), while the details of a regulation can be changed by the empowered authority (the FRA in this case -- State regulations are overseen and modified by the State's PUC). All changes must stay within the limits of what was specified by the enabling legislation, and there must be a hearing and comment solicitation process about any such [proposed] changes.

The demarcation line for diesels is the SN 712 (former WP 712), only the guy who represented the museums at the RSAC process got the build date wrong (he said 1948 instead of 1953).