The scheme is being promoted by Hertfordshire County Council in partnership with London Underground and Network Rail. The aim is to have trains running along the new track by 2015/2016.
Cost of the project, including rolling stock, is estimated at £60m to £80m.
New stations will be provided on the new rail link at Ascot Road, which forms a park and ride site for the area, and Watford General Hospital serving the proposed Watford Health Campus.
The key elements to be built include a viaduct and embankment linking the current Metropolitan Line, 1.3km south of the existing Watford terminus, to the disused rail alignment between Croxley and Watford High Street.
Funding for the scheme was secured in December 2011. A contract notice was published in the EU Official Journal this morning. For further details, see our Contract Leads section.