Signalling question

I have seen in photographs a type of signal attached to station buildings on Colorado NG. They appear as a sideways figure 8, and seem to be a variant of the 19th century disk and crossbar signal. They look for all the world like the key to a clockwork toy. What is the purpose of the signal? They do not seem be lit or visible enough to be a go/stop type signal, and as far as I can make out, most NG trains would be scheduled to call at every water tank anyway..
U.S signalling is very different to British practice, and all I've found on the web so far is about modern Cat1 railroads, or generic historical standard gauge practice.
Education needed, please help