The wagon derailed just after passing through Heworth station on the Sunderland to Newcastle line
An investigation into a freight train derailment has found the track maintenance team could not cope with its workload.
A train part and a section of track at Heworth, Tyneside, had gone unrepaired, the Rail Accident Investigation Branch (RAIB) found.
The train was on the line from Sunderland to Newcastle on 23 October last year when it derailed.
The accident was "immediately caused" by a worn suspension on a wagon.
This made one wheel likely to lift up when it passed over dips on the rail line, the RAIB said.
'Persistent problems'
RAIB said wagon operator Freightliner's maintenance regime had not identified this worn component, though the company had now checked other wagons and modified its maintenance regime.
Network Rail should have imposed an emergency speed restriction because of the severity of the dips in the track but did not, the report said.
The dips had been indentified "many times" but had either not been repaired or the repairs had been ineffective, it said.
Inspectors also found there were "insufficient numbers of track maintenance staff over a long period of time".
"Network Rail's management arrangements did not identify the extent of non-compliances to its maintenance processes, or take earlier action to resolve the persistent problems that were affecting track maintenance in this area," the report said.
Network Rail said it would be issuing a statement.
