Tim O'Toole, chief executive of FirstGroup and former London Underground managing director, said he was "shocked and saddened" by the news of Bob Crow's death.
Mr O'Toole said:
I worked with Bob both during my time at London Underground and here at FirstGroup.
He was a tireless and powerful advocate for his members and was dedicated to the railways and public transport.
Our deepest condolences go out to Bob's family, friends and colleagues.
The late Bob Crow took the trade unions back to their roots and left his members better off - and attracted many critics along the way.
The trade union movement is in shock after the sudden death of rail union leader Bob Crow, who passed away early this morning aged 52
The RMT leader was demonised by Fleet Street, often loathed by commuters, but adored by his members.