Air hoses and Signal hoses?

I have been working on some 1970's era Amtrak Content for Trainz railroad simulator, and I have come something I do not understand.

On the Superliners, Hanging directly next to the Air brake hose is a "signal hose"

I am curious as to the function, and use of these hoses. I have seen them on RPO 60 at the Colorado railroad museum too, along with the steam heating hose. While Superliners do not carry the steam heating hose, they still carry this signal hose.

More over, I am building the SDP40F locomotive. When delivered in 1973, they had steam generators over the rear truck. Some were later replaced with Head end power. I am curious, besides the 6 Multiple Unit hoses, and the air brake hose, would the SDP40F locomotive have a signal hose as well?

thanks for any help about the operation and use of the Signal hose!