La Trochita (official name: Viejo Expreso Patagónico), in English known as the Old Patagonian Express, is a 750 mm (2 ft 5+1⁄2 in) narrow gauge railway in Patagonia, Argentina using steam locomotives. The nickname La Trochita means literally "The little gauge" though it is sometimes translated as "The Little Narrow Gauge" in Spanish while "trocha estrecha", "trocha angosta" in Argentina, is often used for a generic description of "narrow gauge."

The Trochita railway is 402 km in length and runs through the foothills of the Andes between Esquel and El Maitén in Chubut Province and Ingeniero Jacobacci in Río Negro Province, originally it was part of Ferrocarriles Patagónicos, a network of railways in southern Argentina. Nowadays, with its original character largely unchanged, it operates as a heritage railway and was made internationally famous by the 1978 Paul Theroux book The Old Patagonian Express, which described it as the railway almost at the end of the world. Theroux had sought to ride trains as far as possible into southern Argentina but did not include in his adventures the several railroads which were further south than Esquel, presumably because they were not considered operational or with sufficient connection to larger lines.