The work by Kiwi Rail is to replace an old South Island main trunk railway timber trestle rail bridge spanning the Waikouaiti River.
Last week the first stage in the bridge rebuild was nearing completion in a project expected to be capped off late this year.
The bridge renewal near Merton is being funded as part of the Rail Network Investment Programme.
A Kiwi Rail spokesperson said it involved a complete replacement of the current timber trestle construction which carried the rail corridor on steel girders.
"The precise costs are commercially sensitive, but project has an estimated cost of around $18 million," Kiwi Rail said.
The old bridge probably dates back to the early 20th century or earlier.
By late last week the first stage of the work had been completed ahead of a second stage to replace nine sections of the old bridge in the staged approach.
Meantime, the main railway line through the area remains open.
Kiwi Rail said people should still expect trains to be travelling through the area at any time and to be cautious at level crossings.
