The Staatsspoorwegen series 685, better known as NS series 3700, was a 4-6-0 express locomotive introduced in 1910. When the SS needed a more powerful replacement for their 4-4-0 locomotives, the 4-4-2 of series 995 (later NS series 2000) didn't become a success with only five built. So they ordered this 4-6-0 from Beyer, Peacock & Co. from Manchester. Together with further batches from Dutch and German manufacturers, 120 had been completed by 1928.

The locomotives had four cylinders with simple expansion, all driving the first coupled axle. All cylinders were controlled by two sets of Walschaerts gear on the inside. With their four cylinders and drivers of only 1,850 mm (6 ft 1 in), they were also capable of hauling freight trains when needed, especially in war time. In usual express service, they could haul 400 tonnes at 90 km/h (56 mph). Due to their much greater power compared to their predecessors, they got the nickname “Jumbos”.

They quickly became the most important express locomotives in the Netherlands until 1929, when the more powerful series 3900 was introduced. In the thirties, six were equipped with a streamlined casing. Due to space restrictions, these locomotives lost their feedwater heaters and pumps and were equipped with exhaust steam injectors. The casings were removed later, but the injectors proved themselves and were retrofitted to other class members.

After World War II, the NS still operated 100 members of the class. Seven had been destroyed and 13 which had been located to Eastern Germany saw service in the Baltic states. In 1958, 3737 was the last one to be retired and at the same time hauled the last regular Dutch steam train. It was saved from the scrapping torch and from the seventies to 2008, it was operational again.

For the heavy mail trains between Germany and England, which crossed the area of the Staatsspoorwegen, the locomotives of the 300 series with a 2-4-0 wheel arrangement were soon no longer sufficient to manage without double heading. Beyer, Peacock & Co. used their own designs of the 4-4-0 wheel arrangement as a basis and added a trailing axle to accommodate a longer firebox and to keep the axle load below the required 15 tons. However, the trailing axle was not radially adjustable, but only had lateral play.

Although the locomotives could reach high speeds with heavy mail trains, the running smoothness left a lot to be desired. What was even more dramatic was that they dealt damage to the superstructure and thus even had to be stored for several months. With the number 995, the bogie was moved forward by eight inches, but this did not bring a sufficient improvement. As a result, the engines were initially only used on routes with a high axle load limit and the maximum speed later had to be limited to 80 km/h. In the meantime, a superheater has been retrofitted.

When the SS and the HSM merged their vehicle fleet in 1921, the five locomotives were given the numbers 2001 to 2005. There, too, attempts were made to improve the running characteristics and the 2005 was given a series 3700 bogie. Since this also did not have the desired effect, the locomotives were transferred to goods service. Their service ended there between 1930 and 1932. Only the 2005 was converted into a brake van for measuring trains and was used in this role until 1955.