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An accident near Blenheim's Taylor River railway bridge highlights the danger of people being on rail tracks, say police and Tranz Rail.

A Blenheim man, in his early 20s, suffered moderate to serious head injuries on Thursday when hit by a train after climbing up from the river bank to apparently cross the bridge.

The force of the impact flung him onto the bank below.

The man was taken to Wairau Hospital but later flown to Wellington Hospital, where he was in intensive care yesterday.

Police believe that the man was under the influence of an unknown substance at the time of the accident.

They said the accident should not have happened, as the bridge had signs prohibiting people from being on it. There is a pedestrian walkway just a few metres away.

Both Tranz Rail and the police said they could not emphasise strongly enough how dangerous the bridge was to pedestrians.

Tranz Rail spokeswoman Jacky Curson said anyone in the rail corridor was trespassing.

"We just can't reiterate enough how dangerous it is for people to be in this area."