I'm fortunate to have traveled the entire current Amtrak system, plus the following abandoned route segments:
International (Port Huron-Toronto)
Three Rivers (Pittsburgh-Chicago via Akron)
Kentucky Cardinal (Indianapolis-Louisville)
Sunset Limited (New Orleans-Jacksonville and Los Angeles-Pomona via Alhambra)
Southwest Chief (San Bernardino-Los Angeles via Pasadena)
I've also covered a few detour routes, including:
California Zephyr (Denver-Salt Lake City via Wyoming)
Coast Starlight (Los Angeles-Sacramento via Bakersfield and the UP route)
Texas Eagle (Dallas-Texarkana via Gilmer)
The only service I haven't taken is the Auto Train and its arguably unique terminal lead tracks.