Taxpayers would save £1.2 billion a year if the railways were brought back into public ownership, a new report has claimed.
Unite said renationalisation would lead to cheaper borrowing, reduced fragmentation of the industry, and an end to shareholders' dividends.
Transport workers are being subjected to cost-cutting to boost profits, and job insecurity, which is putting pressure on pay, pensions and decent working practices, said the union.
Assistant general secretary Diana Holland told a Unite conference in Brighton: "For too long our public transport has served the whims of the market and shareholders.
"We need an end to deregulation and privatisation of our buses which is resulting in bus wars, the isolation of communities and soaring fares.
"We also need to reunite our railways under public ownership."
Unite urged the Government to bring forward investment in transport as a way of boosting economic growth.
Rail Minister Stephen Hammond said: "Since privatisation, our railways have been a great success story.
"We have seen record numbers of passengers taking record numbers of journeys. Satisfaction, punctuality and safety have all shown continued improvement and a recent European Commission study found that Britain's railway is the most improved in Europe since the 1990s.
"We have no plans to turn back the clock and renationalise the railways when the current system is delivering such strong results."