A "load rating and fatigue assessment report" prepared for Sydney Trains in July and obtained by Fairfax Media recommends strengthening of the upper sections of the Hawkesbury River rail bridge.

The emergence of the report comes after the Labor opposition helped expose crumbling in one of the concrete piers of the bridge, as well as the initial failure of Sydney Trains to conduct recommended repairs.

Fairfax Media subsequently reported transport authorities did not know the strength or carrying capacity of about a quarter of the state's rail bridges – a fact Transport for NSW and Sydney Trains first tried to deny.

The latest report into the Hawkesbury River bridge, over which tens of thousands of people travel every day, examines the upper steel and concrete sections of the bridge.

The report, by engineering consultants SMEC, identifies defects through parts of the concrete, some of which indicate that "corrosion may have been initiated in some locations".

And in relation to the "steel superstructure" of the bridge, the report "identified generally consistent defects across the structure, predominantly surface corrosion and protective coating deterioration".

"Some isolated members and locations have exhibited more severe defects such as full thickness corrosion perforations," the report said.

Asked about the findings of the report, a spokesman for Sydney Trains said there were "no issues with the safety or structural integrity" of the bridge, and that a subsequent inspection in November "indicated that the corrosion is superficial and does not impact on the strength of the bridge".

The spokesman said SMEC's report was commissioned to see whether the bridge could accommodate painting and re-painting – along the lines of what occurs on the Harbour Bridge.

"We are finding out if the bridge can take added load that is required to undertake this work," the spokesman said.

"As an interim measure, adjusted operational conditions will limit use of the bridge to one fully-loaded freight train at a time," he said.

"The interim restriction will not affect customer services."

But the SMEC report obtained by Fairfax Media also recommends strengthening of the bridge. Achieving a "MF" load rating, which is one of the standards used by Sydney Trains, "will require strengthening of select members and connections," the report says.

"The proposed strengthening solution is to increase capacity through plating the existing structure, and connecting the additional plates through huck (blind) bolts," it says.

The report also considers what it would take to strengthen the bridge to meet a heavier "300LA load rating" but says this may be too costly and not worth it.

"This load case occurs only rarely, and so strengthening to achieve an MF rated-load capacity will satisfactorily accommodate the actual applied live loading."

The Sydney Trains spokesman did not say whether Sydney Trains would be conducting this strengthening.

Labor's transport spokeswoman, Jodi McKay, questioned Sydney Trains' explanation about commissioning the report because of plans to paint the bridge, which was already carrying freight and commuter trains.

"They've been less than truthful about what's going on with this bridge for some time now," Ms McKay said.