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.

[2] 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.

The project ran out of steam following ministerial changes and the start of World War I which affected the economy of Argentina and the input of technology and investment required from Europe.

After the end of the First World War, narrow gauge track and facilities were plentiful and cheaper, given their extensive use at the front for supplies and troop movement.

In 1921 it was agreed to lay down the Jacobacci-Esquel line, and also to connect it with the existing private 1,000 mm (3 ft 3+3⁄8 in) gauge railway in the Chubut Valley from Dolavon to Puerto Madryn.

Belgian coaches and freight wagons were ordered in 1922, plus fifty locomotives from Henschel & Sohn, a German company in Cassel (today spelt Kassel).

The bends of the line in the Andean foothills and the slow speed of the network allowed passengers to walk alongside the train on certain sections along the 14-hour journey.

The service was much used for freight through the 1960s and 1970s, contributing to the development of the area, especially the construction of the dam on the Futaleufú River and the growth of El Maitén thanks to the locomotive maintenance operation.

La Trochita also began to decline, due to the improvement of the road network and of trucks and buses, and the difficulties of maintaining a railway so far from the country's capital and the global rail industry.

Theroux's book brought it to wider attention and gave the railway a name – The Old Patagonian Express – which highlighted its timeless appeal both to Argentine nostalgics and tourists.

The maintenance of the original 1922 locomotives is increasingly difficult, due to the lack of parts and expertise, and the remoteness of his base in El Maitén.

The Henschel & Sohn 2-8-2 steam locomotive number 114, built in 1922, encountered unexpected erosion on this embankment, derailed, and rolled onto its side taking all of the vintage wooden railcars along with it.

Nearly 60 passengers (mostly British and American) and crew were onboard this private charter, but, miraculously, there were no fatalities and most people escaped with no or minor injuries (cuts and bruises).