Here's one: the Sprinter line in San Diego. AFAIK the trains run with just the engineer/driver. Of course, since it runs with diesel light rail equipment, not FRA-compliant DMUs, and is time-separated from freight service, it probably runs under FTA rules not FRA.

Besides the evacuation drill, the only real reason I could see for using a conductor on what's effectively a transit line would be to collect tickets, or to sell them if sales at stations are not enforced. Have often thought that the light rail ticket cops in places like Sacramento should be allowed to sell people who don't have a valid one a ticket for a penalty (much higher than normal) price, rather than write up a citation forcing somebody to go to court and (if not dismissed, as many are) pay $200 or more for a fine. Yes, the fine is a deterrent, but let the penalty for the ticket be 4x and RT would cover its cost for the ride, which the regular tickets don't, and the money would come in now rather than weeks or months later after the court date (if the court doesn't just keep it all). But of course then the ticket cops wouldn't be cops they'd be roving conductors and a different union and pay scale might apply...

BTW, regarding the comment about coordinating with buses in the fare zone system, I don't think that will happen right away. SMART doesn't run the buses - other people do. So even if schedules are more or less coordinated, the fare systems are and will be separate. Revenue-sharing agreements will be needed to coordinate at least passes (daily & monthly passes are common on transit systems); in the short term, that may be all that can be hoped for. Even in some single-provider regional transit systems (SacRT, I'm looking at you!), your transfer between light rail and a bus, absent a daily or monthly pass, is another full cash fare. The Clipper Card thing could simplify matters, though: it's effectively a pass that covers the fare division between systems through a 3rd party, though the character of the division (everybody gets a full cash fare? could draw your balance down fast) needs to be understood by the card users.