An Amtrak conductor who works the Texas Eagle between San Antonio and Fort Worth, has told me if an Amtrak run is less than five hours, only an engineer is required in the cab. If it is more than five hours, two people are required up front.
A Fort Worth based engineer works No. 21 from Fort Worth to Austin. At Austin he/she is replaced by a San Antonio based engineer that takes the train from Austin to San Antonio. A San Antonio based engineer operates No. 22 from San Antonio to Austin, where he/she is replaced by a Fort Worth based engineer for the run from Austin to Fort Worth. The scheduled time between Austin and Fort Worth is less than five hours.
The conductor and assistant conductor on Nos 21 and 22 work from San Antonio to Fort Worth or vice versa. The conductor and assistant conductor are based in San Antonio.
As I understand Amtrak’s procedure, if only the engineer is in the cab, the conductor is in constant communication with the engineer. He/she is required to know the location of the train and every signal along the route. As the train approaches a signal, the engineer is required to call it to the conductor for confirmation. If he/she does not call it, the conductor is required to call the engineer for confirmation.