Thieves have stolen 40 detonators from a railway depot in Oxfordshire.
The detonators can explode, causing injury, if they receive a hard blow or come into contact with fire, say police.
British Transport Police said they could be dangerous to children and appealed for anyone with information to come forward.
The detonators were stolen from a Network Rail depot in Bicester overnight on Monday.
They are yellow, flat and round with the circumference of a drinks can.
They are placed on the tracks as warning devices to alert train drivers that engineers or potential hazards are on the line ahead.
Sgt Damon Lewis, of British Transport Police, said: "The detonators contain mild explosives and emit a loud bang when trains pass over them.
"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, and I would urge anyone who has information that could help us with our inquiries to contact us as soon as possible."
Anyone with information should contact British Transport Police on 0800 405040 or speak anonymously to Crimestoppers on 0800 555111.
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