The (, "Fen Railway") is a former railway line that was built partly across what was then German territory by the Prussian state railways. It is now entirely in Belgium, because the trackbed of the line, as well as the stations and other installations, were made provisional Belgian territory in 1919 (permanent in 1922) under an article of the Treaty of Versailles.
This created six small exclaves of Germany on the line's western side, of which five remain. The treaty (not the location of the trackbed, per se) also created one small Belgian counter-enclave, a traffic island inside a three-way German road intersection near which lasted until 1949.
The route is now a cycle way.
The line, which was standard gauge, ran for some across the High Fens to the south of Aachen in a roughly southward direction from Aachen via (the site of the depot), Monschau and to , with a eastward branch from to and . At Eupen it connected with the line to Herbesthal where it joined the Brussels-Cologne main line. At it connected with the Liège-Luxembourg line.
Its part, when and where under Belgian control, was known officially as Belgian Railway line 48.
The line was built to carry coal and iron. The section of the line from Aachen to Monschau was opened on 30 June 1885. The section from to Eupen was opened on 3 August 1887. The link to Walheim was opened on 21 December 1889.
After the First World War due to the Treaty of Versailles, the German Reich had to cede districts of Eupen and to Belgium on 10 January 1920. As a consequence of the new border demarcation, the route of the line now changed several times between the German Reich territory and Belgium. Belgium demanded that the be placed under Belgian administration, as it was of particular economic importance for the towns of and Eupen, and succeeded. On 27 March 1920, a border demarcation commission, which included representatives from France, the United Kingdom, Italy and Japan, determined that the Belgian state should be the owner of the railway line and its stations. At the time this generated six German exclaves, five of which still exist today.
On 18 May 1940 Adolf Hitler ordered that the area be re-annexed by the German Reich, and the was returned to service as a wholly German line on 2 June 1940. However, after the defeat of Germany in 1945, the pre-war situation was re-instated.
After the Second World War, the railway's industrial use declined steadily, so that the line was finally completely shut down and partially dismantled by 1989.
In latter years, until the end of 2001, the line operated tourist services, some of which were steam-hauled. These were withdrawn for want of funds to maintain the line. Part of the track between and is now used by railbikes.
It was reported in 2008 that, with the no longer operational, Belgium might have to return the land the line runs along to Germany, which would result in the reunification of the exclaves with German territory. However, the foreign ministries of Germany and Belgium have since confirmed that the trackbed, even though disused, will stay as Belgian territory and that the German exclaves will therefore remain.
By 4 December 2007, the dismantling of the now disused line had started; by September 2008, the track had been completely removed between and Sourbrodt.
A cycle-way along the route was opened in 2013.
Former enclaves: