Variable Load Valve

Also known as a Retainer. A valve used to vary brake application individually 
on each vehicle depending on the weight of the vehicle, it can be manually or 
automatically operated. In the manual version - used on freight vehicles only - 
a lever at the side of the wagon must be set for the required position.
  
Automatic versions of variable load valves are now often used. A simple version 
is operated by a lever connected between the valve mounted on the car 
underframe and the bogie frame. As the car load increases, the lever detects 
the depression of the car body and valve relative to the bogie and adjusts the 
setting of the valve in direct proportion.
  
On vehicles with air suspension, the lever is used to adjust a levelling valve 
which changes the air pressure in the suspension system so that the car body 
maintains a constant height, regardless of the load. Changes in the suspension 
air pressure are detected by a separate variable load valve and the brake 
application adjusted to suit.
The variable load system can also be used to adjust train acceleration so that 
it is constant regardless of load.