News of a mixed-use Broadgate-style office, retail and leisure scheme, planned for a 6 ha site behind the station and extending over the railway tracks, emerged at the start of the year.
Railtrack is expected to team up with developer Hammerson for the project. The developer owns the lease on four office buildings around the station.
Sources claim that without Hammerson on board, Railtrack would not have received funding from the Strategic Rail Authority to modernise Euston station itself.
Sources close to the deal say an agreement is likely to be concluded between the parties in the summer.
The project is one of a stream of schemes planned for London stations that were hit by the collapse of Railtrack last autumn.
A £300m redevelopment of Victoria station, which was approved by Westminster council last year, was due to go on site next year. The scheme is being masterplanned by architect GMW Partnership.