The structure will replace a Grade II listed wooden toll-bridge at Pont Briwet near Penrhyndeudraeth.
Gwynedd councillor Dewi Lewis said the new crossing, which spans the River Dwyryd, would improve the economy of the area, and overcome the need for an 8-mile detour.
The work will involve detailed design and construction of the new structure, which will carry a single track railway and two lane highway.
Road approaches to the new bridge are to be improved to allow two way traffic, and a 2.5m wide footway/cycleway.
The council wants the successful contractor to commit to proving local apprenticeships and supply chain opportunities.
Work on the new bridge is planned to start in November and be completed two years later.
Firms have until the end of the month to express and interest in the bridge job. The e-tendering address for further information is www.etenderwales.bravosolution.co.uk, reference: pqq_30277 Pre-Qualifiaction for Pont Briwet Transport Improvement Scheme.
Or contact Pat Jones tel: 01286679448, email@example.com
A full Network Rail Principal Contractors License must be held for the duration of the works.