For the Southwest Chief, it would seem the one big question is how Albuquerque would be served (if directly, how much time would it take to divert from the Transcon at Dalies and Belen to travel to/from ABQ and what sort of complications would reversing direction at ABQ entail; if indirectly via a Thruway connection, how much would that hurt ridership). Shifting from the traditional Santa Fe northern passenger route to the southern Transon, the Southwest Chief would lose a number of fairly small stations (La Junta, Lamar, and Trinidad, CO; Dodge City, Garden City, and Hutchinson, KS; and Lamy, Las Vegas, and Raton, NM) but would gain Wichita (city 372,000; metro area 612,000) and Amarillo (city 189,000; metro area 246,000) and whatever other smaller stations it would also serve along the way (I didn't look up to see what stations the San Francisco Chief served up until 1971).
For the Empire Builder, shifting routes would mean the loss of Devils Lake, Grand Forks (metro area 97,000), and Rugby. In this case, it would seem to be largely a state issue (North Dakota to determine whether the cost of keeping service to Grand Forks is worth it).