And rail authorities appear to have been in denial about the gaps in their record-keeping, incorrectly stating on multiple occasions they did know the strength or "load rating" of the state's rail bridges.

Load ratings are used to assess the weights that bridges can carry.

In response to a request using freedom of information laws, Transport for NSW said it did not have load ratings recorded in a Bridge Management System for 123 of 547 bridges that carry trains, or 22.5 per cent.

But in other responses, Transport for NSW and Sydney Trains said even if load ratings were not recorded in a Bridge Management System, Sydney Trains had all the information it needed.

"I have been informed that all Sydney Trains' bridges have load ratings and are inspected," the manager of Transport for NSW's Information and Privacy Unit wrote to Fairfax Media following a request using the Government Information (Public Access) Act.

"Sydney Trains has load ratings for all bridges across the network," a Sydney Trains media spokesman said when contacted for comment.

But following multiple follow-up inquiries from Fairfax Media, Sydney Trains and Transport for NSW conceded these statements are incorrect.

"At the time of the previous GIPA request, of the 123 identified assets, there are 57 bridges maintained by Sydney Trains which we don't have a listed load rating for," the chief executive of Sydney Trains, Howard Collins, said in an email.

"Of these 57, there are 36 which we would need to do further site inspections to derive the load rating," Mr Collins said.

The absence of a recorded load rating does not mean a bridge is unsafe.

But "missing" load ratings has been a topic of discussion between the Asset Standards Authority, an agency set up to monitor safety standards of transport assets, and Sydney Trains for over a year.

Minutes of meetings between the Asset Standards Authority and Sydney Trains show multiple references to determining the "current extent of missing ratings" and discussions over "funding for missing ratings".

In November, the ASA "was to investigate/arrange funding for missing ratings and elevate in ASA/ST and with contract manager; ST to assess missing ratings and examinations to determine any ST actions," according to minutes obtained using the GIPA Act.

Gopinath Chattopadhyay, a chapter chair of the Asset Management Council, the peak professional body and technical society for Engineers Australia, said while the state's bridges may be safe and strong, the absence of load ratings created uncertainty.

"Should those load ratings be in the system? A one-word answer is 'yes'," said Dr Chattopadhyay, from Federation University in Victoria.

"There's no doubt about that. Unless you know your asset you do not know your risk, you cannot plan for your maintenance budget, you cannot plan for your capital investment," Dr Chattopadhyay said.

He said that deriving load ratings was a complicated process that could take a couple of months per bridge. Moreover, load ratings could change over time, for instance under heavy rain.

"If tomorrow ... there is 400mm rain in a couple of hours [and] landslides, how much will the ratings come down?"

Some of the bridges for which load ratings are missing are in some of the busiest parts of the rail network. Load ratings are missing for bridges crossing Duck Creek and Parramatta Road at Granville, where the main Western Line runs, at Milsons Point just north of the Harbour Bridge, and where the East Hills line crosses River Road at Revesby.

Questioned about the missing load ratings, and the fact that Sydney Trains and Transport for NSW had at first denied the missing load ratings, Transport Minister Andrew Constance said he had asked the Secretary of Transport for NSW "to secure an independent civil engineering expert to ensure the process of collecting data for the Bridge Management System is running effectively".

In its responses, Sydney Trains said load ratings were not referred to day-to-day to determine the safety of bridges, but rather for future planning of services. A spokesman said the organisation had a plan to "fully populate" the state's Bridge Management System within 12 months and there was no need to "accelerate it".

Fairfax Media's investigation into the state's missing load ratings followed the emergence of corrosion in one of the piers of the Hawkesbury River Bridge, the rail connection between Sydney and the Central Coast and Newcastle.

Repairs to that bridge were completed in April. But in a report released last week following a request by the Minister, the Office of Transport Safety Investigations cited breakdowns in procurement process and communication as contributing to the delay in repairs.

"The OTSI report exposed some of the real shortcomings in how these railway bridges are managed … and I think that is deeply concerning given the importance of these bridges across the state," said Labor's transport spokeswoman Jodi McKay.