From an FRA point of view, this new rule does not change anything. Commuter railroads in the US have historically used conductors and train attendants.
But it is a really inefficient use of labor. Industry best practice is to have just one crewmember (the driver). Ticket validation can best be handled throrugh random POP inspections. If this rule goes into effect, it will be another obstacle to modernizing passenger rail. DMU operations would be especially problematic. Imagine if the new Marin-Sonoma “SMART” service had to use two crewmembers. Sure they might apply for an FRA waiver, but why create more bureaucratic headaches?