UPDATED 7.26PM: Four railway workers are feeling lucky to have escaped without injury, after their truck caught fire inside a tunnel near Wellington.

The maintenance workers called for help after their truck bust into flames inside the tunnel above the Ngauranga Gorge just before three.

Fire Service central communications shift manager Mike Wanoa says the fire generated a lot of smoke and gas, which is difficult to get out of the tunnel.

Crews have now re-entered to dampen down the scene.

Trains are currently not able to use the Kapiti line.