When did/does the Trans-Mongolian pass through Datong?
Back in 2018, I took train K4 from Ulaanbaatar, Mongolia to Beijing, China. The schedule bolted to the wall of my train (pictured) as well as everything I can find online indicates that the current routing on the Chinese side is Erenhot-Jining-Zhangjiakou (Shalingzi West)-Beijing.
However, our train actually detoured through Datong (lower route), arriving in Beijing several hours after the timetable said we were supposed to, and a casual trawl through Google Books suggests that until at least the 2000s this was in fact the standard route for trains to take. It also was not an unexpected delay, since the arrival time was correct to the minute on our tickets.
When was the route changed? And why did my train take the "old" route in 2018?
Personal guess: something to do with the construction of the Beijing-Zhangjiakou HSR, which was completed in 2019.