Gerry Doherty, 58, is to take early retirement after seven years as general secretary of the TSSA transport union.
He has been a tireless and vociferous opponent of what he sees as the ills of rail privatisation.
Mr Doherty said yesterday: “I think this is the right time to go given all the challenges the union is facing in the coming years.
“We are now in merger talks with our sister union, the RMT, which I hope will come to a successful conclusion next year.”
He went on: “But we are also faced with the biggest attack on our members in a generation with the (Sir Roy) McNulty Report (on rail costs). It wrongly diagnoses... labour costs as the biggest problem facing the industry when John Major’s disastrous privatisation is the real problem.
“We need a new generation of leaders to tackle these issues in both the short and longer term.”
A secret ballot of the union’s 28,000 members will be held in November to choose Mr Doherty’s successor.