Vennbahn

The Vennbahn (German pronunciation: [ˈfɛnbaːn], "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 had the effect of creating 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 Fringshaus.

The line, which was standard gauge, ran for some 75 kilometres (47 mi) across the High Fens (Hautes Fagnes, Hohes Venn) to the south of Aachen in a roughly southward direction from Eupen via Raeren (the site of the depot), Monschau (Montjoie) and Malmedy to Trois-Ponts, with a 20 km (12 mi) eastward branch from Oberweywertz to Bütgenbach and Losheim. At Eupen it connected with the line to Herbesthal where it joined the Brussels-Cologne main line. At Trois-Ponts it connected with the Liège-Luxembourg line.

...
Wikipedia