Exciter for locos, etc.

I am curious how a locomotive signal exciter works. Yes, I know it helps single light locomotives (including RDC's and MU's) to shunt wayside signals and grade crossing circuits. But how does it do that? Some type of RF signal or an electrical current put into the rails? I have also searched the archives and the net, but I have not seen any technical detail on how it works.

I never knew about this device. I do know that movements of fewer than a specified number of axles have certain restrictions. My company requires an absolute block for fewer than 12 axles, for example. Some require approaching crossings and absolute signals prepared to stop.

One day I was running a single light engine on the main. We were approaching a block signal. Our signals are approach lit. The signal was going on and off. I thought it was a signal malfunction, so I called the dispatcher. He said that I was disappearing from his CTC display. Since I wasn’t shunting, I made sure that crossing gates came down.