Operator Metlink said there was a potential issue and Worksafe New Zealand said there were two incidents of elevated nitrogen dioxide levels in passenger trains that triggered the change in schedule\ reports Stuff.

The railway tunnel through New Zealand's Remutaka Range, was opened to traffic on 3 November 1955. For a long time, it was the longest tunnel in New Zealand.

https://www.stuff.co.nz/dominion-post/news/wairarapa/125045382/passengers-put-on-bus-after-poisonous-gases-detected-in-remutaka-rail-tunnel