Doug,
The horizontal position for both UQ and LQ Semaphores gave the most restrictive aspect for either (Stop or Stop-and-Proceed aspect).
However, the LQ had a couple of drawbacks:
1. It was at least theoretically possible for a heavy accumulation of ice on the blade to cause it to drop to the lower position, which could give a "False-Proceed" aspect, arguably the most dangerous of all signal-system failures.
2. Many LQ Semaphores - including the ones used by the PRR - gave only 2 positions of the blade - and, hence, only 2 colors - for each blade. A typical set of aspects for an Automatic Block signal requires 3 positions. So the LQ required 2 signal arms/heads for each ABS signal.
The PRR began using LQ Semaphores in its earliest ABS installations. At certain locations (up the hill to Paoli, on the East Slope, etc.) the PRR evidently desired to keep train headways to a minimum because of heavy traffic. The normal 2-block, 3-aspect configuration (in today's nomenclature Clear, Approach, Stop-and-Proceed aspect sequence) did not provide sufficient Safe-braking Distance (SBD) with such a configuration in the downhill direction without causing speed to be limited to below what was desirable. So the railroad went with a 3-block, 4-aspect configuration (Clear, Approach Medium, Approach, S&P sequence) in the downhill direction. With the LQ Semaphore, this required 3 arms.
The PRR began replacing LQ Semaphores with UQ in the early-1900s. The UQ Semaphore allowed one arm to be eliminated in each ABS signal. The UQs remained in service in many places until the 1940s or so. Of course, the PRR developed the Position-Light (PL) Signal about 1915 for the Paoli electrification, had it perfected by about 1920, and adopted the PL as standard. The res is, of course, history.
See Westing's "Pennsy Steam and Semaphores" for more information.
Best regards,
Lin Bongaardt
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