Strathspey Steam Railway needs a crossing on the River Dulnain so it can offer journeys between Aviemore and Grantown-on-Spey.
The group runs locomotives along 10 miles (16km) of line between Aviemore and Broomhill.
It needs three more miles of track (4.8km) to link up with Grantown.
Railway enthusiasts have hired a powerful crane to lift the lower sections of a new bridge into place across a river in the Highlands.