Lisa Grant works for South West Trains. Like her heterosexual colleagues, she wants a train pass for her girlfriend. Her employers said no - so she took them to the European Court. Kathy Marks reports
This morning, at 10.30, 15 scarlet-robed judges will file into the chamber of the European Court of Justice, a granite building on a hillside overlooking the city of Luxembourg. The simultaneous translators will switch on their headphones, and Case C249/96 will commence. Seated in the second row, her stomach in knots, will be Lisa Grant, a 29-year- old railway clerk from Swindon.

The proceedings have evoked a degree of interest rare for the Luxembourg court, which usually arbitrates on esoteric points of European Union trade law. For one thing, Lisa is represented by Cherie Booth QC, on her first high-profile outing since her husband, Tony Blair, became prime minister. For another, this is an important test case, one that challenges residual prejudices and could cost British employers dear.

Lisa's demands are fairly modest. Her job is handling telephone enquiries at Southampton Central station and, like her colleagues, she has a concessionary travel pass for the rail network. She wants a similar pass for her girlfriend, Jill Percey, so that they can take inexpensive holidays together. Her employer, South West Trains, has refused, saying that only spouses or "common-law" partners of the opposite sex qualify.

After an industrial tribunal hearing, the case was referred to the European Court, which has been asked to clarify the relevant law. At issue is whether public-sector employers may legally discriminate against their homosexual staff. A ruling in Lisa's favour, which is expected by many observers, would outlaw such discrimination in all workplace-related matters, including recruitment, promotion, benefits and pensions.

Soft-spoken and diffident, Lisa is an unlikely champion of gay rights. She and Jill, a nurse at the Royal Hampshire County Hospital, lead quiet lives in Eastleigh, a small town near Southampton. Until this dispute, their most militant action was attending the annual Gay Pride festival in London. More than once, they have insisted that they are not "banner- waving lesbians".

It was a sense of injustice, and injured feelings, that drove Lisa to take on her company. "What they're saying is that our relationship isn't meaningful," says Jill. "It makes us feel like second-class citizens. It's very hurtful."

Jill, 37, does the talking, since Lisa's contract prevents her from speaking to the media. Jill is also the more forthright of the two, and ferociously well versed in the legislative minutiae. Last weekend, as the couple collected their thoughts before leaving for Luxembourg, she darted around the terraced house that they share with three dogs and two cats, rummaging through dozens of bulging ring-binder folders on the case.

The document that elicits fits of giggles from both women is a booklet outlining the equal opportunities policy of South West Trains. In it, the company states: "We are committed to ensuring that all individuals are treated fairly irrespective of disability, race, gender, health, social class, sexual preference, marital status, nationality, religion, employment status, age or membership or non-membership of a trade union."

As Jill talks, Lisa fiddles with the slim silver ring that she wears on her wedding finger. In 1993 the couple pledged their commitment at an "affirmation ceremony" in their home attended by friends and family, including Jill's elderly parents, who are devout Baptists. The women had met the previous year, in pouring rain, at a rounders tournament. They hit it off immediately, and a few months later Lisa left Swindon and moved in with Jill.

In early 1995, she filled in a form requesting the travel pass for Jill, and arranged to swear an affidavit stating that they had been in a relationship for two years. It was at that point that the personnel manager, Alan Warner, took her aside and told her some hard truths. She was startled. Everyone in her office was aware that she and Jill were a couple; they had attended Christmas dinners together without turning any heads.

At the industrial tribunal in May last year, South West Trains made its position clear. It did not deny that it was treating Lisa unfairly; on the contrary, it freely admitted it. What its barrister argued, quite simply, was that neither English nor European law prohibits discrimination against homosexual workers.

Why, though, has the company, which inherited the case from British Rail, fought it so vigorously? Certainly, the pass is of considerable value: it gives 75 per cent discount on all standard tickets as well as 10 free 48-hour journeys a year. But the number of staff in Lisa's position must be tiny; since she and Jill set up a lobby group called Fare's Fair, they have been contacted by only 70 aggrieved gay rail employees, none of whom work for South West Trains.

Jill has another theory. Reaching for yet another folder, she reads out the defence that Lisa's employers will present to the Luxembourg court. In it, South West Trains explains that Lisa and Jill "do not have the sort of relationship which the company's regulations aim to encourage". And in a passage that would not be out of place in a papal address, it states: "A relationship between persons of the same sex cannot be regarded as equivalent to a marital relationship between a man and a woman, based on the traditional concept of the family and the concern to promote the procreation and rearing of children."

It is a curious thought, the employer as promoter of family values, guardian of moral standards in its workers' private lives. But if there is one thing that this case has highlighted, it is the widespread corporate ambivalence in the face of shifting modern relationships.

Even within the rail industry, policies diverge. Of the 27 private operators that replaced British Rail, seven give travel passes to partners of the same sex; Stagecoach, which now owns South West Trains, does so in its bus sector. British Airways allows staff to nominate a partner of either sex, saying it recognises "the variety of circumstances that exist".

Lisa and Jill, meanwhile, are struggling to come to terms with their new-found status as gay icons. They are, they say, private people who never made an issue of their sexual identity until this dispute. Reluctant campaigners, they are happiest when walking their dogs in the New Forest.

"We're like two separate couples," said Jill. "In our normal lives, we get up in the morning, feed the animals, go to work, come home and watch TV. Lisa reads a lot; I knit. In our other lives, we're lesbian activists. We work on the case, do television interviews and have complete strangers come up to us in the street."

In Luxembourg today, Ms Booth's strategy will be two-pronged. She will argue that Lisa's employers have contravened both Britain's Equal Pay Act and the European Union's Equal Treatment Directive. She will compare Lisa to Rob Potter, a colleague who was given a pass for his long-term girlfriend, Sheila Carey.

South West Trains, for its part, will maintain that is not guilty of discrimination provided it treats its gay male and female staff in a similar way - what Ruth Harvey, Lisa's solicitor, calls the "equality of misery" argument.

Lisa and Jill will have to wait a year for the court's ruling. Tomorrow they will return from Luxembourg by train. Perhaps next time they take a trip together, it will be on equal terms.