Following up on Prof. Martens' immediate thought, I think that Pittsburgh (40.44° N, -79.99° E) is as far West as Auto Train can expect a passenger draw. Yes, I know I live in Chicago (42N, 88W) and have used it twenty four times over the years, as I have had, but no longer, friends in the Wash DC area (retired; reloed).
Likewise, draw a line through the 80th Meridian, and that too represents the limits of a potential AT market.
Now my thoughts on the chorus around here wanting to extend such to a point on the Corridor; really forget it. True, I know first-hand over the past fifty years, the build up around Lorton, where, when ATC inaugurated service, the biggest "tourist attraction" ("uh, not exactly" by choice) was the DC "correctional facility" located there. I-95 was a quite adequate four lanes and the 95-295-495 interchange (395 then not designated) "perfectly navigable even by Cadets".
But, even running a "Corridor AT" along the B&O, RDG, and CNJ to a New York area destination is fraught with issues. First, a "slot over" the Potomac River "Long Bridge", then clearances along the freight line through the District. Next there is Landover and the grade X-ing of the PRR (don't you think the present day Amtrak Train Dispatcher "just loves it" when Chessie has a train ready to X there?). So to address all these constraints (second Long Bridge span, DC clearances, a Landover flyover) are simply too much $$$$ when the alternatives for Mr. Train Guy et al is simply allow enough time to drive to Lorton - and most of the AT retiree passenger base has plenty of that!!!