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20181020_094145

John, it is a snifter valve. I always wondered why the models have them painted gold. As seen on the 460, the valve has perforated sheet metal, which is brass on the 460, don't know if it was always brass. But it would get blackened dirty  so. .

I suppose that behind that brass there is a sprung plate that is pushed outward closed when steaming, and pulled open by the vacuum when drifting.

I specifically took that pic and cropped it to a close up because I had just learned of its function as well. 

From Wikipedia:

When the driver shuts off the steam to the cylindersof a steam locomotive while it is in motion, the moving pistons could create a partial vacuum in the cylinders. This would give rise to two problems. Firstly, the pumping action would absorb energy and prevent the engine from coasting freely. Secondly, when the exhaust valve opened, soot and cinders from the smokebox could be sucked down the exhaust pipe and into the valve chest or cylinder, causing damage. (The exhaust is open to the smokebox because in normal running the exhaust steam is sent through the blastpipe to draw the fire and eject the combustion products from the chimney.) These problems are avoided by using snifting valves to allow air to be drawn into the cylinder. On railways which did not use snifting valves, drivers were instructed to keep the regulatorslightly open when coasting to avoid the creation of a vacuum.

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