ATS vs ATC

Trains Magazine General Discussion
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Actually the CNW ATC system does not require a heavy brake application to comply with a high speed restricting signal. If the train speed is already less than 40 mph or only slightly above, then a light to moderate service application will reduce the speed to less than 40 mph within the 6 seconds, there will be no penalty. If the service brake application can reduce train speed to less than 22 mph within 70 seconds, there will be no low speed penalty. In the real world however, if a heavy train (say a loaded coal train) is moving at 60 mph and a high speed signal occurs, only the immediate movement ot the automatic brake handle to Suppression can forstall the penalty before the 6 seconds are up which leaves the engineer no choice but to place the handle into Suppression position that also makes the automatic brake produce a straight-away full service brake pipe reduction. All the engineer can do now is to bail off and brace for a strong slack run-in.

There is another way that the high speed penalty can be prevented. Apply the independent brake above 30 psi. This works but the risk of flattening locomotive wheels is high. When locomotives first employed high friction compositon brake shoes CNW used a J1.4-14 brake cylinder relay valve that produced a maximum of 64 psi at full independent. UPRR used a J200 brake cylinder relay valve that produced 90 psi at full independent. When UPRR locomotives were run through on CNW they would rerurn with flat spots on the wheels. It was found that CNW engineers were using a 35 - 40 psi independent application to prevent the high speed penalty but they did not realize that the trailing UPR

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The CN’s ex-IC/ICG/CC Iowa Division mainline had ATS between Waterloo and Ft. Dodge (possibly out to Tara as well) but the Chicago Central had that taken out by 1986 or 1987.

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Harmon never had to resolve the problem. The issue just never existed in their equipment design. US&S’s inductive antenna circuit would “ring” at the proper frequency. That made the antenna (cab signal bars) very sensitive it the “right” frequency. The Harmon circuitry just read the signal “flat” across all frequencies. If the track circuit had the wrong carrier frequency, the US&S equipment would never see it. The Harmon systems would pick up everything but then filter the output for the carrier frequency. Conrail went through some huge birthing and growing pains with Harmon developing their cab signal and LSL system. It was worth it in then end. The Harmon Ultracab system proved to be better AND much cheaper than US&S in the end.

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That spounds like it might make an interesting article on the development and de-bugging of the technology - history plus managing the technological risk/ uncertainties, Murphy’s law, cost effectiveness, the effects of different design philosophies, etc. Maybe such an article would be a little less sensitive to those involved, now that it looks like it will all soon be superseded by PTC ?

  • Paul North.