POLICE have warned that lives will be lost if vandals continue to dump lumps of concrete on a railway line.
An express train was nearly derailed by concrete put on the railway line at Loudwater, last month.
Concrete has also been dropped from bridges and thrown at trains.
PC Donald Elliot, of the British Transport Police, who is home beat officer responsible for the track between Marylebone and High Wycombe, said: "There could obviously be very serious accidents, especially as on parts of the line the track is on a high embankment and a derailed train could come down causing loss of life."
The driver of an express train was shocked when it hit concrete put on both rails at Loudwater at 5.20pm on April 4.
The impact had a 'ripple' effect right down the train, which weighs about 70 tons.
The train was travelling at 75 miles an hour from Beaconsfield towards High Wycombe.
The train stopped and the driver used a radio to report the incident.
The concrete blocks involved were about three feet long and two inches deep.
Between March 31 and April 4, there were at least six incidents reported by drivers where objects were put on the line or stones were thrown at trains.
Checks have been made on the undersides of trains for damage and a door was found dented.
PC Elliot said: "There's always the risk of rocks going through the driver's screen or hitting a passenger."
Police have appealed for anyone who saw youths or others on bridges or on the tracksides to call the police.
In these incidents, Thames Valley Police first attend and then the Transport Police are alerted.
One of the worst areas is around the Whitehouse Tunnel, near Whitehouse Lane, Loudwater.
Fires have also been lit in the tunnel.
"School holidays seem to be one of the worst times for this and whoever is responsible needs to be made to realise the terrible consequences, even loss of life, these incidents could have," said PC Elliot.
% Information should be passed to the British Transport Police control room on 0171 387 0354
Picture shows PC Donald Elliot with a concrete block vandals left on the line
Converted for the new archive on 30 June 2000. Some images and formatting may have been lost in the conversion.
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