GN S-1 4-8-4 Boiler Thingy

[GN_Fan]

I have a brass model of a GN S-1 that I've had for decades.  I know most things on a steam locomotive, but this model, and photos of the prototype, have a thingy on top of the boiler that leaves me clueless.  It's behind the steam dome and pretty much right over the rear driver and looks like a micro-sized grain elevator with a sloped conveyor belt off of the forward end.  Anyone have a clue what this is??

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[Semper Vaporo]

I am not real sure about this, but I believe that is part of the "Boiler Low Water Detector".  The housing at the back end (upper end of the sloped pipe) is a mechanical switch/valve that will release steam to a whistle in the cab if the sloped pipe gets longer due to heat.  The sloped pipe fills with water and stays relatively cool as long as the low end is in water in the boiler.  If the low end gets exposed, the water will drain out of the sloped pipe and allow it to get hotter and thus expand in length and open the steam valve to allow the whistle in the cab to sound off, warning the crew of the low water condition.

[GN_Fan]

Thanks a lot for your informative answer to something that has bewitched me for more than a few decades.