I went to York on an express that was packed to its metal gills and running late, but still it seemed to me a good way to travel because I knew that David Hare's latest play was about the railways and I myself was once a keen train-spotter who took particular delight in taking the numbers of the puffers at King's Cross. I travelled back to London holding nervously on to my seat and wishing I was going by helicopter, canal boat, anything but a train.

The thesis of The Permanent Way, which reaches the National Theatre in January, is that we're paying far too heavily for a privatisation that even Margaret Thatcher did not want. We are doing so not just through fares that are scandalously high, given the unreliability of services, but through danger and disaster. And the 100-minute piece which makes this case is disturbing enough to merit comparison with any of the state-of-Britain plays that became known as the Hare Trilogy.

But as presented by the touring company Out of Joint, The Permanent Way is very different from the fictional Racing Demon or semi-fictional The Absence of War.

Mainly, it consists of selections from Hare's interviews with everyone from the former head of Railtrack to Nina Bawden, who lost her husband at Potters Bar. You learn what it's like to hear that your dead son smelt like "human barbecue" and how it feels to come bouncing off the rails at 100mph.

Hare prepares us carefully for these horrors. Max Stafford-Clark's excellent nine-person cast, crammed together and lurching over the set's few chairs, begin by evoking the lunatic stoicism of the British public. Then it's over to Whitehall mandarins and investment bankers who coolly anatomise and regret a privatisation that would seem comically botched and illogical -imagine a restaurant whose cooks, waiters and washers-up work for different companies -were it not for the consequences as Hare's cast evokes them.

Southall: a driver untrained in using warning systems that anyway weren't operating. Ladbroke Grove: a red signal whose near-invisibility was well known.

Hatfield: a faulty but neglected rail. Potters Bar: catastrophic points. And so to deaths, inquiries that achieved little, distressing conflicts between survivors' and bereaved groups, and lasting pain.
The voices include outraged parents, the incisive, outspoken transport policeman who was quietly sidelined after Ladbroke Grove, the executive whose thoughts at Hatfield seem only to have been for his company's future, and John Prescott, who emerges as the smug, stupid, devious, self-serving spokesman of a deeply untrustworthy Government.

Is Hare fair? Isn't Stafford-Clark's scariest back-projection, a wild, splintering derailment, an appeal to the emotions rather than the mind? I myself interviewed the wife of one of the 120 who died at Orpington in 1957, and she blamed British Rail, with reason.

Nationalisation doesn't guarantee safety. But Hare has still made a serious, provocative, dramatically gripping contribution to an argument of urgent interest to us all.