Two teenagers are being hunted by police after a second attack on a Wivenhoe-Colchester train in as many days.
The train was standing at Wivenhoe station when the vandals damaged the glass before getting off and running away.
Det Sgt Roger Napier of Colchester police said Wednesday's attack occurred at 3.10pm.
It happened just a day after a train windscreen was smashed by youths throwing stones at it.
PC Neil King, of British Transport Police, said smashing train windows from inside or outside was dangerous.
"It is not only the damage to the train, it is the injuries which could be caused," he said.
"Anybody on the train could be hit by shards of flying glass. Throwing stones at the front of the train could injure or affect the driver."
He added: "It is a very stupid, mindless thing to do."
Juliet Sharman, of First Great Eastern Railways, said a company campaign told children and teenagers about the dangers of playing near tracks or causing vandalism.
"We target schools where we believe there are problems," she said. "We have a very hard-hitting poster campaign and we send drivers out to schools to talk to children and explain the dangers."
The youths wanted in connection with Wednesday's attack are both thought to be about 16. One is described as 5ft 8ins tall with fair hair in a curtain style. He wore jeans and blue and white trainers.
The other had dark hair and wore a black leather jacket.
Anyone with information about either attack should call Colchester police on 01206 762212 or Crimestoppers on 0800 555111
Converted for the new archive on 19 November 2001. Some images and formatting may have been lost in the conversion.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article