Communication Signals

By Fred Boland, Machinist 1928-1980
Bayshore Shops, Southern Pacific Railroad

Locomotive 3199, a commute GP40-2, built in 1974, was sent into Bayshore to see why the communicating signal sounded off by itself at odd times. Sooner or later the engineer would have acted on it, believing it was instructions from the conductor.

It is surprising what was so standard yesterday is not known or understood today, except by the real old-timers. What is a communication signal system? This is a system developed about a century ago when the conductor pulled a bell rope that went through the passenger cars and tapped a bell in the cab. So many clangs of the bell indicated whatever the conductor wanted. Without going through the whole list, as an example, three taps when running told the engineer to stop at the next station. If stopped, three taps meant back up.

Sometime in the late 1800's an air system was substituted for the bell rope. Since the air brake gradually replaced the handbrake through the 1890's there has been a 1-1/4" pipe down through the cars to control the brakes. On passenger equipment a second pipe of 3/4" diameter similarly runs down through the train. This pipe is charged to 45 PSI in steam and 55 PSI in diesel. A pot about 10 inches in diameter and 8 inches in height containing a reservoir of air and 2 diaphragms, called a "Type C Locomotive Signal Valve", is used. A pressure reduction raises the diaphragms, admitting some air from the reservoir to the space between the diaphragms to which an air whistle is connected. Stopping the reduction allows the feed, or reducing valve to restore the pressure, letting the diaphragms fall back into place.

Apparently the rest of the railroad has been so long in freight only, in spite of an Amtrak locomotive going through occasionally that the shop men never knew, or have forgotten passenger trains. Certainly the store department was ignorant. Bayshore store department had no "whistle pots". Luckily, I found that a cocked spring in the check valve was the culprit.