The vehicles built to Diagram 1/146 formed the bulk of British Railways 21t Hopper fleet. The vast majority of them were constructed with only a lever operated hand brake, but the Modernisation Plan of 1955 called for stock to be fitted with Automatic Vacuum Brakes. This process was slightly hampered with hopper wagons, as there was no space under the chassis suitable for mounting of the vacuum cylinder or the vacuum pipe. The solution was to mount the vacuum cylinder on one of the end platforms and required a re-design of the end supports to the hopper. Subsequently the vacuum pipe was fitted along the top of the chassis, running through the gaps available in the side bracing. Due to the large number of vehicles under construction by several builders, the fitment of vacuum brake equipment was carried out somewhat piecemeal, with some vehicles not receiving all the equipment when new to traffic.

The vehicles featured in this pack belong to the three final lots built to Diagram 1/146. Lot 3158 from Birmingham Railway Carriage and Wagon, Lot 3159 from Gloucester Railway Carriage and Wagon and Lot 3160 from Hurst Nelson Ltd. of Motherwell. Nearly all were built in 1958, with construction of the last vehicles of Lot 3158 running into January 1959. When built, the hoppers all had the full compliment of vacuum brake equipment installed and were fitted with roller bearing axle-boxes.
Coded as HOP21 VB when new, these vehicles were painted in the standard Bauxite scheme for fitted stock. A new style lettering appeared in 1964, with the vehicle code, load weight, tare weight and vehicle number all grouped together in a box displayed on the left hand end of the body. A decade later, TOPS coded these vehicles as HTV.

The dia. 1/146 21t HTV Coal Hopper Wagon Pack for Train Simulator includes

• Two variations of freight brown livery
  
• Clean and weathered versions
  
• Loaded and empty variants with suitable physics
  
• Four different type of data panel/lettering - randomly assigned but can be overridden
  
• Loaded versions have five different load profiles randomly assigned
  
• Animated handbrake lever
  
• Dynamic builders plates
  
• Intelligent taillamps

The pack includes a user manual with further details about the rolling stock and the customising of some of the features when using the wagons in scenarios.