Located at Milepost 90.3 on the former Rio Grande Moffat Tunnel route is a wonderfully analog system to help prevent inattentive train crews from derailing eastbound trains on an especially sharp 20mph curve. Despite the later deployment of ETMS PTC on the territory this holdover from the days of yore has remained in place and is observable by persons traveling on Amtrak California Zephyr Train 6.
The Colorado Rockies have a geologic feature known as a mountain "park" which are wide flat valleys surrounded by generically mountainous terrain. The west slope of the DRGW Moffat Tunnel route alternates between impossibly tight canyons with a line speed of 25mph and these wide flat parks with long stretches of 60+ mph running. The mountain park near Kremmling, CO allows for 60+mph speeds between mileposts 82.2 and 86.2 on track that is close to tangent. At milepost 86.6 this abruptly ends in a wheel squealing 20mph curve as the line turns into Byres Canyon.
To mitigate the risk of an inattentive crew hitting this curve at full speed a radio warning was installed at the milepost 90 automatic signal location. When an eastbound train is detected an audio message about the upcoming 20mph track is broadcast 4 times over the road channel. The distance between the warning and the curve is close to 3 miles and is more than sufficient for the engineer to control the train's speed.
The milepost 90 signal was replaced around 2015 when ETMS PTC was added to the line, however the radio message was left in place and today it serves as reminder of the sort of analog innovation that took place in the analog era like the timer based cab signal drops that were installed at similar high risk locations on the NEC in the 1980's.
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