Re: Signalling question

I don't think any of the above links actually state the purpose of said signal.

This signal is a Train Order Signal. The most common form of train dispatching in the US in the 19th and 20 centuries was "Time Table & Train Order". In this system you have a published timetable that trains are run by. If you deviate from this time table or add additional trains, "train orders" are issued to cover their operation, change in operation of scheduled trains. This can be a very complicated system that I won't even begin to explain. But in short, if the dispatcher decided to change something in a train's train orders, he would telegraph a way side station and give the order to the station agent. The agent would copy the order, then place the train order signal to "Stop" to indicate to the approaching engine crew, they had new orders to pick up at the station. When there were no new orders for a train it would be placed in the "clear" or proceed position.