One-man operation is the industry standard practice for regional and commuter services. However that isn’t the case for Denver RTD — which still uses both a driver and conductor:
Recent staffing issues for the Regional Transportation District’s (RTD) University of Colorado A-Line have caused cancellations of trips, leaving travelers waiting at Denver International Airport (DIA) for hours. Since January, 1,548 A-Line trips have been canceled, according to data from RTD. About 80% of those cancelations – 1,290 in total – happened because there wasn’t a second crew member available to be on the train. One reason for this is that the A-Line requires two people to be on board the train during a trip.
Thanks to automation, two-man operation became obsolete decades ago. To make matters worse, RTD work-rules require the second crew member to be an armed guard (i.e. rent-a-cop) — which limits the pool of applicants.
Systemic Failure 
Multiply this by a thousand and you’ve got the current state of the NYC subway and regional rail lines: the unions successfully fought off any serious suggestion of OPTO (never mind full automation, lol) for years, and now between COVID and people simply leaving for better jobs the MTA is regularly running major lines with 12+ minute headways during rush hour, and 30+ minutes off-peak, simply because they don’t have the personnel. Just an amazing own goal all around.
But Denver’s wrinkle of requiring the conductor to be a rent-a-cop is extra special, and I think maybe we should be very quiet about that lest the NYPD get wind of another infinite overtime grift.
2 man operation at least has a historical reason in NYC. Isn’t RTD’s system brand new?
Picking up trash in a horse drawn wagon once was the standard in NYC, as well. We are pathetically behind the rest of civilization here when it comes to rail.