Looking at pictures on Google maps, that stretch of track is arrow-straight and the land is table-flat, so speeds were probably up over 50 mph in both directions.
Not all trains or track segments have the automation and safety features of the most modern signaling systems. I'm guessing it was a human or electronic version of either "asleep at the switch" or "blew past the signal".
In the olden days, switches were all operated by hand. Those guys have all retired, of course, replaced by electric switches that can be operated from a big control center in Omaha.