The FRA requires a certified engineer on each train.

The FRA requires a certified conductor on each train.

BUT - In creating the conductor certification rule the FRA specifically discussed who the "conductor" on the train was. When MRL was created, they made a deal with the BLE that eliminated the UTU, so they run an engineer and "engineer's helpers". There are also some shortlines that run engineer only trains. The rule states that someone on the train must be certified as conductor, but that person can also be the certified engineer. So a train can be engineer only, have a qualified brakeman or helper (not certified as a conductor) - and still be in compliance as long as the engineer is also a ceritifed conductor.

As far as Amtrak or other passenger operation, under the present FRA rules they could in fact run engineer only, or have the conductor ride the cab. I believe the current BLET union agreement allows Amtrak to run a single engineer on the head end as long as the scheduled running time is less than 6 hours. The engineer can't copy any authority while at the ocntrols of a moving train, so Amtrak typically assigns an assistant engineer in TWC/DTC/OCS territories. If the alertor fails, a second rules qualified person has to be in the cab.

Years ago when the host railroads supplied the operating crews, Amtrak created the position of "On-board Chief" to be in charge of the on-board service staff and be able to resolve many of the customer service or service employee issues before involving the conductor. Amtrak still assigns one on long-distance trains. The smaller/shorter trains typically have a Lead Service Attendent who might work by themselves or have subordinate helpers or car attendents.

As mentioned, while train crews can't work more than 12 hours, the on-board staff typically work from origin to destination. On most long distance trains, the conductor and ACs will take care of assisting people on or off, and getting them settled into their seats letting the car attendents get some rest. A good car attendent will let the train crew know what to expect, and will often still get up if there is a busy station. There are too many car attendents that are nothing more than slugs riding a train that seem to be nowhere to be found leaving the Conductor and ACs to do their jobs also.

Notice how Amtrak requires Conductors and ACs to be in uniform, but the service staff typically is dressed casually? The reason for this is to give the train crew an "air of authority", while making the service staff "friendly". Letting the service staff run the train will create several issues. First - the service staff would be responsible for handling all tickets and cash fares. Second - The service would have to work 24 hours per day. Third - The conductor is still in charge of the train, and some states have historic laws on the books specifically recognizing the conductor's authority. Fourth - We would have a conflict between the UTU losing jobs (possibly eliminating all the ACs?) and the service union gaining jobs. Though I did hear a rumor that a few years ago when union negotiations where at an impass, Amtrak was considering eliminating all UTU covered positions leaving the operating crew as an engineer and assistant engineer letting the service employees take care of everything within the train.

As far as layovers - I have also been involved where the scheduled layover was too short to accomodate a late running train to allow a train crew to be legally rested. It really isn't that easy to schedule trains because there is so much more to consider that just crew efficiency. Many trains are scheduled to go through the best scenery or higher traffic stations during the daylight hours. Connecting trains also have to considered, as well as where they can add extra time to the schedule to account for the insane delays caused by the host railroad. How the equipment turns or can be serviced also comes into play. Plus - allowing for any work windows or dodging the rush of other freight or passenger trains on the same line.