These streamlined single carriage DEMUs were built by Allan of Rotterdam in 1953-54. They were extensively used on the branch lines in Friesland as well as on other non-electrfied secondary routes. They also operated the Maastricht-Liege shuttle. Originally painted blue (hence the nickname), they were later repainted dark red. They were largely replaced by new Wadloper DMUs in the 1980s.
The class DE-2 DMUs were similar, but with two (articulated) carriages. The DE-2s were later refurbished and repainted into yellow NS livery, but the DE-1s retained the dark red livery until withdrawal.