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By Greg Richardson, general director-Operating Technologies

The cab signal aspect display located inside the locomotive cab, left, has been replaced with PTC computer technology seen in this modern locomotive cab.

Dec. 21, 2022, marked a technological milestone – the last of Union Pacific’s locomotive cab signal systems were discontinued, marking the end of the use by UP of a technology that served us for nearly 100 years.

Greg Richardson, general director-Operating Technologies

What exactly are cab signals? No doubt you’ve seen wayside signal systems in use alongside train tracks. They’re like “traffic lights” that tell a train crew how to proceed, slow down or stop. Locomotive cab signals serve the same purpose. From wayside devices, electronic signals are transmitted through the rails and received and displayed via colored lights mounted inside the locomotive cab. This signal technology emerged in the early 20th century, promoted as “a superhuman engineer” and “an invisible guardian.”

By 1922, federal regulators ordered most Class I railroads, including Union Pacific, to install the nascent cab signal technology. Union Pacific installed cab systems on over 2,000 miles of its railroad, generally on the east-west main line between Chicago/Kansas City and Ogden Utah.

Cab signal systems remained a safety standard until the Rail Safety Improvement Act of 2008 mandated Positive Train Control (PTC) systems be installed on high-traffic Class I railroad main lines. Using computer technology, PTC provides the same cab signal information inside the locomotive cab but communicated directly from the wayside to the lead locomotive by data radio or cellular communications. In addition to exceeding cab signal systems performance, PTC provides train crews with current location, speed limit in effect, speed limits at locations ahead, and information about work zones, mileposts and switches.

With the ongoing expansion of PTC and thanks to a federal waiver that allowed trains to operate with PTC in lieu of cab signals, Union Pacific began to phase out the use of cab signals in 2016. However, regulators still required cab signal systems remain active and that a cab-signal-equipped locomotive be in the lead on every train just in case of a PTC failure.

By the end of 2019, PTC had been installed on all cab signal lines – an even more significant milestone achieved by unprecedented cross-functional department coordination. And in 2021, PTC reliability warranted regulators to approve the full discontinuance of cab signals in mid-2022. And now Union Pacific Mechanical and Signal departments have begun decommissioning cab signal systems altogether.

Aside from superior safety and operating benefits of PTC, the following are achieved through discontinuance of use of cab signals and associated equipment:

Using the rails as a communications path to the locomotive was a clever and practical solution for safety and train control systems in its day. However, PTC and modern wireless data communications provide a quantum leap in capability for the next generation of signaling technologies and safety. One immediate opportunity is the elimination of wayside signal lights in favor the direct communications to the PTC computer display. Signal lamp removal will reduce power consumption at wayside signal locations and eliminate some maintenance costs.  

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