An equal pay claim has been launched on behalf of women managers at Network Rail.
The Transport Salaried Staffs Association (TSSA) said the women were being paid between £3,000 and £4,000 a year less than their male colleagues for doing the same job.
The claim involves 30 women, but could cover 3,000 if the case is won, according to the TSSA.
The union will stage a protest outside Network Rail's annual meeting on Friday as part of its campaign against bonuses being paid to executives.
Assistant general secretary Lorraine Ward, said: "It is simply outrageous that a taxpayer funded firm should reward those at the top with huge amounts while at the same time discriminating against thousands of their own staff who happen to be women.
"As a soon to be fully public sector firm, it should reward everyone the same regardless of gender. Perhaps the male bosses just don't get the full picture sitting in their comfortable men-only first class gravy train."
A Network Rail spokesman said: " Network Rail is committed to equal opportunities and fairness and equity in its pay. We are working hard, in partnership with our trade unions, to address any legacy imbalances within our pay structure. Those discussions continue.
" No-one at Network Rail received a bonus for train performance this year as our targets were not reached.
"Recognising that large bonuses are no longer an acceptable form of incentive for Network Rail, a new, substantially reduced bonus scheme, is to be put before the company's equivalent to shareholders (called Members) at its AGM this week which proposes to slash directors' maximum potential bonuses from 160% of salary to 20%."