With the loading gauge no obstacle and the light weight of the load, conversion the WMX class wagons to a triple deck car carrier was an obvious improvement. WMX 34039 was the prototype  conversion and became class WMB in 1977.   The vehicles selected for the conversion were the remaining second series WMX now WMFX wagons.   All nine wagons (34021- 34028, 34030) were converted in 1989.  34029 was brought into line as a WMGF in 1993. In 1994 all 10 wagons were hired to NRC as RMWY. However 34023 and 34030 were reclassified RMGY in the same year (possibly in error).