The Rail, Maritime and Transport (RMT) union has blamed some Tory losses on a long-running dispute over the role of the guard on Southern rail.
The RMT claimed Conservative losses in seats along Southern lines like Croydon, Eastbourne and Brighton, as well as close-calls in areas like Amber Rudd's constituency of Hastings, were down to the "axing of guards and the impact on safe, accessible and reliable services".
Together with the train drivers' union, Aslef, RMT has been in a row over the role of conductors for months, facing multiple failed negotiations.
"These results prove that the toxic Southern rail franchise was a game changer in key seats along the routes served," said Mick Cash, RMT general secretary.
"RMT is demanding that the axing of the guards is reversed and the union will harass Theresa May and the transport ministers in her minority government every step of the way as we step up the fight to put safety and access to services before private profit and greed."
The union said passenger groups backed its claims.
A spokesperson for Southern said: "People will see these claims for what they are. The constituencies cited by the RMT were all marginals with tiny majorities and therefore vulnerable to small electoral changes. There were similar results in many parts of the country where passengers have not suffered from the union’s industrial action."
RMT has said it will ramp up its campaign for public ownership of the railways as "the minority Tory government collapses into chaos and recrimination".