Federal law requires a conductor on every train. I suppose the argument could be made to put the conductor on the locomotive, but that would displace the second engineer on trains so equipped. You still need an employee on the train, an authority figure, with a radio. OBS employees cannot be trusted to handle the conductor's duties inside the train. They sleep during the night, and even when awake they aren't exactly the sharpest knives in the drawer if you know what I mean.

So where is the inefficiency? For starters, they have too many crew bases for the engineers to make their runs in less than 6 hours. Amtrak employs a ton of extra management to look after these crew bases which negates much of the cost savings.

Second, the scheduling of the trains is very inefficient. T&E crews layover at crew change points for up to 24-40 hours and this forces Amtrak to pay heldaway wages after 12 hours, plus additional hotel costs, plus meal allowances. As of a few years ago it was costing Amtrak $15,000 per day in heldaway wages/meals/hotels that wouldn't otherwise be necessary if the schedules were adjusted to minimize the crew layovers. Fixing this problem would make the train scheduling very unpopular with the passengers, so it's never been taken very seriously.