A teenage boy has lost the sight in his right eye after a railway detonator exploded in his face, police said.
The 15-year-old suffered severe facial injuries after at least one detonator was thrown on to a bonfire and exploded on Saturday night, British Transport Police said.
He was taken to Southend Hospital and is expected to undergo surgery later.
The boy had earlier been playing with two friends at a skate park in Belton Way, Leigh-on-Sea, in Essex.
It is believed they took a box of detonators, which are used as fog signals on the rail network, from a nearby railway depot.
Two 17-year-old boys have been arrested on suspicion of theft and are being held at Southend police station.
The incident was reported to police at about 10.50pm.
Detective Constable Michael Bute, from British Transport Police, said: "This is a tragic incident which has had very serious consequences for the youngster involved.
"It does serve to highlight the dangers of trespassing on to any part of the rail network that is not meant for public access."