Re: San Juan Connections...

A signal line is an air filled pipe running the length of a passenger train. It is smaller in diameter than the train pipe (automatic air brake) with a smaller glad hand. It works by the trainman opening a signal valve to exhaust air and lower the pressure in the signal pipe. A device in cab of the locomotive cab responds to a drop in pressure by opening another port that allows compressed air to operate a signal whistle.
In the pre-radio days this allowed the train crew to signal the engineer all kinds of stuff. From the simple highball to a final stop signal when the tail car is on the platform. Also to indicate that the crew had a passenger to get off at the next flag stop. Most rule books included the signals for signal line whistle.
Brian Norden