Five horses have been killed after being hit by two trains between Middlesbrough and Darlington.
It is not yet known how they got on to the railway tracks.
The incident happened at a site near Durham Tees Valley Airport, just before 7.30am yesterday (Tuesday).
Two other horses were involved in the incident but did not get on to the line and have been taken into care by the RSPCA. Words: PA
The police said officers and staff from the Environment Agency attended but it was not clear how the horses came to be there.
A British Transport Police spokesman said: "We were called to the railway line near the airport station after reports a number of horses had been struck by two trains.
"Colleagues from Durham Police and the Environment Agency were also in attendance.
"Five horses were killed. Two other horses from the scene are now in the care of the RSPCA.
"It is not clear how the horses came to be on the line."
A bus replacement service was put in place straight after the crash but trains were later said to be running normally.
Leanne Plumtree, spokeswoman for the RSPCA, said: "The RSPCA has had an officer at the scene of this terrible incident this morning.
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