Forty railway detonators, which warn train drivers that engineers are on the line, have been stolen in Warwickshire.
Shrapnel from the detonators, which were at a site on a line in Rugby, may seriously injure someone if they were set off, British Transport Police said.
The items, which are small, yellow coloured discs, were taken from a location near Parkfield Road overnight.
The detonators are safe if handled correctly, and are designed to be triggered by an impact.
As well as warning about the presence of engineers, they also warn train drivers about other potential hazards on the line.
Sgt Martin Smith, from British Transport Police, said: "They could be very dangerous in untrained hands, particularly young children or teenagers who may see them as a source of fun or entertainment.
"We are obviously concerned for the safety of the individuals who have the detonators, regardless of their age."
Anyone with information is asked to contact British Transport Police or Crimestoppers.
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