Under the Hours of Service Law, when the clock strikes twelve, "that's all folks". The train, either class or service, had best be on a siding when cinderella becomes a pumpkin. The only exception to Hours of Service is an emergency; I'm willing to bet, but I've been away from the industry too long to be authoratative, that if a passenger train's HEP was Bad Order that would be an emergency and accordingly the train could proceed to the next place passengers could be safely detrained.
As noted earlier, Amtrak, under its existing Agreement with the Employees, can avoid assignment of an Asistant Engineer if the scheduled run if less than six hours or if reasonable and practical conditions preclude such (example Auto Train; no opportunity to "sit on the throne"). Therefore, this in itself becomes an incentive to establish shorter crew districts, but on the other hand, shorter districts may require more Meals and Lodging (remember the bunkhouse no longer is "suitable lodging"; hasn't been since the 1964 Agreement); also may require more employees which translate into more fixed "head" costs such as RRTA Tier I, RRUI, and the "biggie', Health & Welfare.
Further, we must acknowledge that it is a bit more stressful to "drive" the train, than to sit back in the cars handling revenue etc, even though the "guy in back" is the "skipper'.
Lastly, there is historical reason for the longer Train as distinct from Engine crew districts. For passenger Train crews (C&B) the minimum pay (the famous "Day's Pay" comprosed 150 miles; all others 100. Therefore if passenger Train crews only ran Albany-Syracuse, 130 some miles, there would be unused miles that the men (yes, MEN, back then) would be paid for anyway. All other positions running Albany Syr would be paid 30 some "overmiles'.