Throttle Valve's on GWR Locomotives.

[daylightguy]

Hey so I was watching a video of someone operating a GWR Hall class locomotive on the Great Central railroad and I remember them mentioning something on the throttle called first and second valve. Can someone please explain this feature or its advantages and was this a thing only exclusive to the Great western or did other railroads have this on their locomotives?

 

[edh6]

Most steam loco regulators have two valves. The first is smaller (the starting valve) and is used to get a more controllable amount of steam for starting out whilst the second (known as the main valve) is opened up when the engine is going and greater power reserves are needed for acceleration.

The regulator uses a sliding section which gradually uncovers the two valves, one after the other, the main valve is uncovered by simply moving the regulator handle further.

There are other types of regulator, but this is probably the most common.