The Croxley Rail Link will be an extension of the London Underground Metropolitan line from Croxley, to Watford Junction via Watford High Street.
The council is now looking for a partner to deliver the new 4.5km line.
Work will include construction of a new viaduct and bridge and embankment improvements plus new stations at Ascot Road and Watford General Hospital.
Consultant Mouchel is working up designs, which are being developed with the aim of starting construction in Spring 2014.
The winning firm will have to co-ordinate with LUL contractors who will carry-out signalling and existing station improvements under a separate deal.
Up to eight contractors will be invited to tender and firms must express their interest by June 7.
For more information contact Hertfordshire County Council Strategic Procurement Group, County Hall, Hertfordshire Zoe Upson SG13 8DQ Hertford UNITED KINGDOM firstname.lastname@example.org or visit www.supplyhertfordshire.g2b.info/hpf/