I've got several...
My daily commute is along the BNSF (BN, CB&Q) mainline from Kansas City to Lincoln. Coal unit trains, grain unit trains, manifests, business specials, rail, the "circus train" (gandy dancer variety) ... if you have a long view, it'll come by.
Downtown Kansas City, at Union Station, is another place.
Reno, NV, on the UP (SP) mainline.
For sheer beauty, no one can beat Refugio State Beach in Santa Barbara County California. I had one spot where I could see freights and the Daylight through about a 2 mile view vantage
Omaha too, at the Durham Western Heritage Museum or the Amschack.