The JX hopper wagons were introduced between 1963 and 1977. They were a pneumatic discharge hopper, using "Air Slide" type equipment: porous floor with air pumped through it to allow the powder to flow like liquid.
JX wagons were numbered 1 - 108 and the class was recoded to VPCX from 1979.
Numbers 1 - 58 were built 1963 to 1971. Numbers 59 to 108 were built 1976/1977. The first 58 wagons were fitted with a three pipe air feed arrangement. These were modified to a single feed supply from the mid 1970's.
The material for two wagons in the JX project of the late 1960's was used to build two flour wagons which were coded _FX_.