Back when the Crescent went beyond Atlanta only 3 days per week and the B'ham to Mobile portion had become a bus, I rode it from Washington to Mobile with the bus beyond Atlanta. We had a full to the last seat bus to B'ham, but only 6 people or so beyond B'ham to Montgomery and Mobile. Maybe that was 8 to 10 out of B'ham and only 6 beyond Montgomery. Obviously that has been a few years,. I don't think a train out of B'ham to Montgomery would be worthwhile. As a guess the run time would be somewhere around 2h15m to 2 1/2 hours. That would give a run time from Atlanta to Montgomery of around 6 1/2 to 7 hours, which would be twice or more of the driving time.
If you're really going to get through pax from the East then Amtrak should use the route through Opelika.
About 176 miles. but above 4 hours run time if I remember the 1960's schedule for the Crescent. Maybe a logical southern terminal for a second New York - Washington - Atlanta train that would have an early morning arrival in say about Charlotte NC southbound and in the range of Philadelphia to New York northbound.
By the way, with the current available New York to Washington times, we could have a New York to Atlanta day train without being rediculously early on one end and rediculously late on the other.