Mindless yobs could have caused a fatal accident after putting a concrete block on the train line near Bennerley Viaduct.
British Transport Police are appealing for information this week after a train hit the block.
Pc Andrew Spiby said: “I cannot over-emphasise enough the dangers associated with this type of crime, it is vital that we find those responsible.
“Thankfully this time, through luck, no-one was injured, but we may not be so fortunate if this happens again.
“Trains travel at high speeds and there can be some serious consequences when they strike these objects deliberately placed on the line.”
He said officers were making enquiries in the area and viewing CCTV in a bid to find those responsible.
The incident happened at the footbridge at the viaduct about 6pm on Wednesday April 24.
“We are urging anyone who was in or around that area to contact us if they saw anything suspicious or have any information that may help us with our enquiries,” he added.
“If you know who was responsible, or have any information that could assist our investigation, we want to hear from you.”
Anyone with information should call British Transport Police on 0800 40 50 40 or Crimestoppers anonymously on 0800 555 111.