Visitors are no longer welcome in train cabs after a driver was so distracted by a conversation with colleagues he forgot to stop until 15 minutes past a required stop.

A Transport Accident Investigation Commission report on the incident said the empty coal train continued for nine kilometres while the driver chatted with workmates, The Dominion Post reported.

The incident near Reefton in Buller has pushed KiwiRail to change its policy about visitors to train cabs.

"... onboard discussion most likely distracted the locomotive engineer, meaning that he forgot the limit of his track warrant as it approached and passed," the report said.

In August last year the train was travelling from Lyttelton to Ngakawau, 31km north east of Westport, when it stopped to pick up two KiwiRail employees.

They began talking and soon after, the driver forgot to make a radio call - the train drove through Reefton, where it was required to stop.

Eventually the driver realised his mistake, stopped the train and notified train control.

The KiwiRail employees had been instructed not to distract the driver.

"The cab pass-holders were there to learn about the route and the operation. The only person who could give that information in this case was the locomotive engineer, so the risk of distraction was always going to be present," the report said.

KiwiRail employees on familiarisation trips must now be escorted by someone else familiar with the area.