Amtrak. You can sit back and watch the country roll past.
Shortest & most scenic: NYC-CHI (Lakeshore Limited) then CHI-EMY(California Zephyr).
A bit longer and also very scenic: NYC-CHI, CHI-SEA (Empire Builder) then SEA-EMY(Coast Starlight).
Another option after NYC-CHI is CHI-LAX (Southwest Chief) LAX-EMY (Coast Starlight).
Longest and probably least scenic NYC-NOL (Crescent) NOL-LAX (Sunset Limited) LAX-EMY Coast Starlight).