It lost its border status on 10 January 1920, however, as a result of changes mandated in the Treaty of Versailles, which left Herbesthal more than 10 km (6.2 mi) inside Belgium.

The Rhine Railway Company ("Rheinische Eisenbahn-Gesellschaft") obtained the king's agreement in 1837 to the construction of a rail link from Cologne to Aachen, crossing into Belgium at the frontier on the edge of Herbesthal.

The line from Cologne as far as Aachen was inaugurated in 1841, but construction progressed more slowly through the hills to the west of Aachen, above all because of the time needed to build the Ronheider incline, the 700 metres (2,297 ft) long Busch Tunnel and the 200 metres (656 ft) long Hammer Rail Viaduct over the Geul River south of Hergenrath.

The entire length of line from Aachen to Liège entered into service on 24 October 1843, and quickly became part of one of the most important international rail routes in western Europe.

During the 1920s up to 1,000 mail bags were processed here daily, and it was only after the introduction of so-called mobile sorting offices, using dedicated rail-cars, that the postal facilities at Herbesthal station were removed.

The goods station was expanded, the depot receiving two extra "ring-form" parking slots for locomotives accessible using a new turntable.

There was also a so-called "Prince's Waiting Room" reflecting the importance of international rail travel generally, and of the frontier station more specifically, even for monarchs and their extended families.

[4] Between the British and German (and other) royal families there was a large and proliferating network of kinship connections which would ensure regular use of the "Prince's Waiting Room" during the closing years of the nineteenth century.

As well as this, the station was important for increasing volumes of freight traffic, even though some westbound trains were routed to avoid the southbound stretch because of the gradient of the Ronheider incline south of Aachen.

In the Edinburgh Evening News a reader provided a vivid account of her problem-free crossing of the frontier at Herbesthal from Germany into Belgium during the early morning hours of 2 August 1914.

Eupen and Malmedy were transferred from Germany to Belgium, along with the adjacent enclave of Moresnet which had enjoyed a strangely ambiguous political status since 1816.

The actual frontier moved a short distance up the line to the southern entrance of the (subsequently renewed) Busch Tunnel.

Directly following the war there were relatively few "stopping trains" carrying passengers beyond Herbesthal into Germany using the new German frontier control station near the hamlets at Astenet and Hergenrath.

During the 1920s Herbesthal increasingly lost one of its important pre-war functions, with the train operators now switching between Belgian and German locomotives not at the frontier but at Aachen.

Belgium was again invaded by German troops in May 1940, although developments in motor transport during the previous two decades meant that the generals were no longer quite so dependent on the rail network as they had been for the First World War.

Before the war the locomotives housed and maintained at Herbesthal had been those used for local passenger and goods trains, but during the later 1940s there were fewer of these, initially because of austerity and then during the 1950s because of the growth in road transport.

Herbesthal now became the home deport for the "Pacific class" NMBS/SNCB Type 1s, built in the late 1930s but still used for the prestigious international luxury trains, and during the 1950s the most powerful high speed steam locomotive used in Belgium.

An added advantage of choosing Aachen was that many international passenger trains were already held up there while being reconfigured, as blocks of carriages from the directions of Rotterdam and Brussels were split or merged for the next part of their journeys across Germany.

Work began on electrification of main lines in 1961, and this immediately involved some of the international trains headed towards Brussels and Ostend being rerouted onto the Montzen Route to the north, thereby avoiding Herbesthal completely.

Passenger services along the branch line to the east, via Eupen to Raeren in Germany had already been ended on 28 March 1959, so that by 1961 Herbethal had also lost its function as an interchange station.

Although passport and customs checks were by now increasingly conducted by officials as the train continued on its way, there remained a need for administrative offices on each side of the frontier.

Through the 1920s and 1930s, however, the heavy luxury international express services from Ostend were generally hauled by locomotives maintained at the depots in Liège and Brussels.