UPDATED 9.57am: Two Hutt Valley train surfers could face fines of up to $10,000 if they're caught.

A motorist shot a video of the pair hitching a lift on the back of a train yesterday.

The stunt's being condemned by Tranz Metro.

General manager David Shepherd says they'll be pushing for the maximum possible fine.

"Anyone who's on the train corridor, on railway tracks and outside any of the designated passenger areas is effectively trespassing."