The early Lego Train sets were developed in Germany. Many of the sets in the grey era were modeled after or inspired by prototypes in use by the Deutsche Bahn (yellow/red for electric trains, black superstructures over red running gear for steam locomotives). A few sets have been based on other prototypes – for example the French TGV trains and Austrian "Krokodil" engine. More recently, sets modeled after United States trains have been released, reflecting the introduction and subsequent popularity of LEGO Trains in the US: the Santa Fe Atchison, Topeka and Santa Fe Railway (10020) and Burlington Northern Santa Fe (10133) Locomotive.