Compared to Europe american trains are very big, a typical train weighs 6000 - 7000 t, is about 1 mile = 1.6 km long and consists of about 100 freight cars. Typically the train is hauled by 3-4 locomotives, but on steep grades the trains often have »helpers« in the end of the train.
On my first 2 trips I hadn't planned were to see trains. It was more "look, there's a train, pull over and take a few pictures". On my later trips my railroad adventures have been more planned; I have found descriptions of good railfanning locations on the internet. Visit www.frograil.com, www.dhke.com (Chicago), Railroad Crossings and Cajon Pass Group.
I have good experiences along the BNSF mainline through Arizona and the Mojave Desert in California and also Tehachapi og Cajon. Here you'll normally find lots of trains, sometimes up to a train in each direction every 10 minutes.
Also UPs Transcon through Nebraska and Wyoming has a lot of traffic, and in Chicago you'll find the railroad hot spot of the United States with mainlines, yards and junctions everywhere.
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