There is no federal requirement that I am aware of that requires two qualified persons on a crew or in a locomotive cab. I have on shortlines, and I know of other shortlines, that run one person crews, and now the engineer also has to be qualified as a conductor. If it is in remote control, there has to be an automatic reporting systems that if the "man-down" alarm is tripped the message is sent to someone actively monitoring it.

The last I knew, Amtrak by union agreement, is allowed to run engineer only in the cab as long as the scheduled running (not counting station stops) time is less than six hours. There is a rule requirement that I believe is federal that the engineer at the controls of a moving locomotive cannot copy authority, so in dark territory an assistant engineer is generally required. The dispatcher usually is able to wait for the train to be stopped at a station for the engineer and conductor to copy unexpected or annul slow orders.

I think the only reason railroads are not really fighting the investment in PTC is that they have every intention of using it to justify one person train crews for at least most general freight trains. I think you will also see train operations becoming substantially automated with the "engineer's" job becoming nothing more than to hit a big red button and feed the dog.

If you think about it, in the last 50 years we have gone from at least five men on a train crew down to two persons soon to be one, but the regulations haven't really kept up with what technology has allowed.