La Trochita, (El 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 Narrow Gauge" in Spanish. It 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.

In 1908, the Government of Argentina planned a network of railways across Patagonia. Two main lines would join San Carlos de Bariloche in the central Andes with the sea ports of San Antonio Oeste on the Atlantic coast to the east, and Puerto Deseado on the coast to the south east. Branches were to be built to connect the mainline with Buenos Aires Lake (connecting at Las Heras) and Comodoro Rivadavia (connecting at Sarmiento). Colonia 16 de Octubre - the Esquel and Trevelin area - would be connected via a branch line to Ingeniero Jacobacci. The whole network would connect to Buenos Aires via San Antonio Oeste.