The campaign for Heathrow Hub, a proposed transport interchange at the world's second-busiest airport, is set to gather pace next month.
Arup, the engineer that first proposed the hub, is understood to be meeting with representatives from interested parties, including Crossrail and Network Rail, in early December. Arup is believed to be gathering support ahead of the publication of refreshed plans later in the month.
A rail-industry source said that the proposal has been heavily altered, meaning that the previous estimated budget of £4.5bn is now irrelevant and could be higher. A supporter of the project said that there will be a push for the private sector to provide the financing on the 200-acre site.
Investors would make their money from increases in land values and property development around the hub.
"There is much greater clarity of the nature of the interchange," said the supporter. "This will be a multi-modal interchange with a two-and-a-half hour travel time to Paris."
The hub would join up with the proposed High Speed Two (HS2) scheme to link London to Scotland, as well as improve travel times to Heathrow from major urban areas such as Southampton and Oxford.
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