Watch the evolution of Amtrak from 1971 to 2011 – Greater Greater Washington
Amtrak system map - also shows Amtrak bus (Ambus) service
I looked at
Amtrak-2021-Corridor-Vision-May27_2021.pdf looking for patterns.
Expansion of existing service: Capitols, San Joaquins, Cascades, Heartland Flyer (DFW - OK City), Hiawatha, Lincoln, Illini/Saluki, Wolverine, Pere Marquette, Blue Water, River Runner, Vermonter, Downeaster, Empire Corridor, Adirondack, Keystone, Pennsylvanian, Richmond - Roanoke Norfolk Newport-News, Carolinian, Piedmont
Added to existing long-distance service (some with Ambuses): Central Coast, LA - Indio, LA - Tucson, DFW - San Antonio, Houston - San Antonio, Milwaukee/Madison - Minneapolis, Chi - Indianapolis, Boston - Albany, Cleveland - Buffalo, Atlanta - Birmingham, Jacksonville - Orlando - Tampa, Orlando - Miami, Tampa - Miami
Former service (some with Ambuses): LA - Las Vegas, Phoenix - Tucson, Houston - DFW, Front Range (Denver - Cheyenne part), Minneapolis - Duluth, Cleveland - Detroit, Indianapolis - Louisville, New Orleans - Mobile, Atlanta - Charlotte
Ambus service: Milwaukee - Madison (Ambuses are Chi - Mad), Milwaukee - Green Bay, Chi - Quad Cities (from Galesburg), Chi - Rockford, 3C+D (Ohio), Boston - Concord, New Orleans - Baton Rouge
Across the border (VIA Rail): Detroit - Toronto
Existing commuter-rail service (LIRR): NYC - Ronkonkoma
None: Rutland - Burlington, NYC - Reading Scranton Allentown, Western NC, Southeast NC, Atlanta - Nashville, Birmingham - Montgomery