The rebuilding of the damaged Ngaruawahia rail bridge late this year is expected to stop children hitching rides to school on passing trains.

Teenagers in the Waikato town have taken to jumping on to the trains, which must slow to 10km/h as a safety precaution after the bridge was damaged two years ago by a derailment.

They call the 200m ride across the Waikato River to Ngaruawahia High School "high balling" or "the wiggles." Last month police and teachers described it as a tragedy waiting to happen.

However, Tranz Rail yesterday announced it would rebuild the damaged half of the bridge later this year, allowing trains to continue through the area at the normal speed of up to 100km/h.

A Tranz Rail spokeswoman, Nicola McFaull, said the rebuilding of the bridge was not connected to the train- riding craze.

"The bridge repair will prevent this, but it has to do with the fact that when it was damaged in 1998 they put in temporary repairs," she said.

"The trains will be too fast for them to jump on."

Disruptions to main trunk line passenger services would be kept to a minimum during the 10-week rebuild, which starts in October, she added. She could not release the cost of the repairs.

The bridge was damaged in March 1998 when seven wagons on a freight train derailed, causing the shutdown of the main trunk line for a week.

Ms McFaull said it had taken the two years since the accident, in which no one was injured, to investigate what work was needed on the bridge.