Hardwood beams were replaced with steel columns on five of eight spans before the Covid-19 pandemic halted the replacement programme.
It has not yet resumed since seasonal passenger services returned to operation on a limited basis.
However, plans were in place and some strengthening had been carried out in the meantime, the company said.
The Wingatui Viaduct is the tallest wrought iron structure in Australasia and travelling across it is a highlight for passengers on the scenic train trip through the gorge.
The Dunedin City Council, which owns Dunedin Railways, has yet to work out what level of ratepayer subsidy might be needed to boost upkeep of the gorge line as part of an effort to rebuild services after the pandemic.
However, Dunedin Railways commercial operations general manager Rebekah Jenkins said last week the three remaining hardwood spans on the Wingatui bridge would be replaced in the next three years.
A 2016 email from DCL Consulting managing director Dave Charters showed a five-year replacement programme was envisaged back then.
Wholesale renewal of the rail beams was recommended.
The five-year timeline was also conditional on an annual detailed inspection of the rail beams.
Ms Jenkins said this week the suggested timeline of five years was an assessment produced in pre-Covid operating conditions.
Dunedin Railways had been implementing DCL Consulting’s recommendation, completing five of the eight spans in three years, before the council put the company in hibernation mode in 2020 amid Covid restrictions.
Ms Jenkins said the replacement programme of bridge beams was paused at that time.
After restarting operations much less frequently, and having replaced the most significant beams on the Wingatui bridge, Dunedin Railways worked with DCL to check the bridge’s safety, she said.
This did not reveal problems, but they decided to strengthen the remaining hardwood beams, doubling them up with steel channels, she said.
Ms Jenkins said this was done because Dunedin Railways was in hibernation and it also bought the company some time.
The five-year timeframe from 2016 had been an estimate based on condition of the beams, she said.
"However, not all rail beams were as bad as others."
The bridge was being monitored closely, she said.
Mosgiel-Taieri Community Board chairman Andrew Simms, who had raised safety concerns, was far from reassured.
"The failure of any one of these beams would result in an unimaginable disaster," he said.
Mr Simms took little confidence from a "patch-up job" on the remaining hardwood beams.
Dunedin Railways plans to restart a trip from Dunedin to Pukerangi on October 5.
Both Dunedin Railways and DCL Consulting have offered assurances the route is safe.
