Vandals almost derailed two commuter trains after dumping concrete and tree branches on a busy railway line.
Transport police are hunting the "mindless hooligans" who obstructed the Glasgow to East Kilbride line during the evening rush hour.
In less than two hours, the commuter line was blocked twice by debris and only the quick-thinking of the drivers prevented a tragedy.
One of the trains was left with thousands of pounds worth of damage.
In the first incident, several large lumps of concrete were left on the tracks, near Busby station, at around 5.25pm on Monday.
The driver, who was heading to Glasgow Central, had to slam on the brakes to stop the train from hitting the blocks and the service was delayed as maintenance teams removed the obstruction.
Less than two hours later another train, heading in the same direction, collided with heavy branches and suffered extensive damage at Busby Viaduct only a short distance away.
Nine passengers were onboard the East Kilbride to Glasgow service at the time of accident, which happened around 7.20pm, but no-one was injured.
Twenty services were cancelled and other trains faced considerable delays.
Officers are carrying out door-to-door inquiries at homes near the stretch of track and will study CCTV footage in a bid to trace those responsible.
British Transport Police today appealed for witnesses and urged members of the public to come forward.
Detective Chief Inspector Jim McKelvie said: "This was an incredibly dangerous act of vandalism and although fortunately no-one was injured the consequences could have been so much worse.
"These mindless hooligans that endanger passengers and rail staff alike put themselves in grave dangers as well."
The area of Busby isn't known as a hotspot for vandals, but last month thugs almost derailed a passenger train at another location after leaving two lumps of concrete on a railway track.
The train was travelling at around 50mph when it hit the blocks just yards from Inverkip station, Inverclyde.
The driver of the 8.50pm Weymss Bay to Glasgow service slammed on the brakes and was forced to stop following the collision.
The impact damaged the train's front wheels but it managed to complete its journey after a short delay.