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.