A bit of a shock for the driver as well.
A number of years ago I spotted a Landslide ready to slip, it was massive just near the exit/entrance of a tunnel , it was on the 125mph section on the main line that goes to London .
It was before the days of cab telephones on trains, so I reported to the guard of our train and told him I work for British Rail Research department, He alerted the Driver who made a un-scheduled stop at a station to report it, the line was closed for months to make it safe.
I was thanked for reporting it and saving many lives .
Another I reported was on the new (then) Derby bye-pass road.