The report - Who Says There Is No Alternative? - published today follows Network Rail’s announcement to carry out a strategic review into the prospect of opening a number of new high-speed lines.
It says that tens of thousands of new jobs would be created and air pollution levels would fall if planned investment at Heathrow airport was switched to new high speed rail lines.
An environmentally friendlier alternative to tens of thousands of short-haul flights could be offered, boosting the economy and leading to new jobs, said the RMT.
The RMT said it was the first union to look for an environmentally acceptable alternative to Heathrow expansion.
RMT general secretary Bob Crow said: "This report shows that high-speed rail can provide a win-win solution for the economy and the environment.
"Our report underlines the urgency of the review announced by Network Rail and for the government to ensure that it acts swiftly to reap the clear environmental and economic benefits that high-speed rail can bring.
The report highlights that 20 per cent of flights from Heathrow are already served by a viable rail alternative and one in five more are to places where rail is the potential alternative
Labour MP John McDonnell, chairman of the union's parliamentary group, and MP for Hayes and Harlington which includes Heathrow said: "This report proves conclusively that there is a rail alternative to Heathrow expansion and undermines totally the argument for a new runway and sixth terminal.
"It should provide the basis upon which the Government thinks again."