A tank that burst through the floor of a packed commuter train during a derailment could have taken off "like a skyrocket".
Monday's derailment, the second in Wellington in four days, came as the train approached Wellington railway station.
It caused major commuter delays and all 26 ageing Ganz Mavag units in Tranz Metro's fleet were temporarily taken out of service for urgent checks.
Former KiwiRail engineer Randall Prestidge, who specialised in vehicle maintenance, said yesterday that the fleet was due for a full mechanical overhaul as long as five years ago. That was put off, then cancelled, when Greater Wellington Regional Council decided to invest in new Matangi trains from Korea.
Only one Ganz Mavag - distinctive for being painted green rather than blue - was overhauled, at a cost roughly half that of a new train, Mr Prestidge said.
"Now we have got to wait for 2015 for new trains to arrive. So the poor old Ganz Mavags just soldier along."
Mr Prestidge, who took voluntary redundancy from KiwiRail in 2009 and has spoken in support of the organisation since, alleged that maintenance work on Ganz Mavags was minimal.
Instead of replacing gear with new equipment when it failed, spare parts were being swapped between units. "They have laid up some of them and taken bits off them."
Brackets for the braking system air-reservoir tank that came off the train on Monday were not due to be overhauled, but any problems with them would have been spotted during the process, he said.
The tank, which weighed about 100kg, was filled with high-pressure air and, if its top had been knocked off on Monday, it could have "rocketed around like a skyrocket", he said.
Regional council transport chairman Peter Glensor said the first Ganz Mavag train was overhauled in a pilot programme. When this was found to be "not as successful as we had hoped", the council accelerated its purchases of Matangi trains.
Last year it bought 96 new units, allowing it to retire the old red English Electric trains and 15 of the worst-performing Ganz Mavags. The council hoped to replace all the old carriages in the next two years.
Mr Glensor said, as far as he was aware, there was no suggestion from KiwiRail or maintenance engineers that it was unsafe to run the remaining 26 Ganz Mavags without the recommended mechanical overhauls. While the overhauls would not go ahead, a "very rigorous" maintenance programme was in place for these engines, he said.
KiwiRail chief executive Jim Quinn said the Ganz Mavag trains were being "maintained for their purpose".
"The investigation . . . now needs to run its course so that we deal with the facts in a proper and considered way."
KiwiRail and the Transport Accident Investigation Commission are both investigating the accident. The New Zealand Transport Agency made inquiries and decided not to start a third investigation, saying the scope of the other two was sufficient.
Meanwhile, KiwiRail has confirmed that a Matangi train derailed in the Wellington station yards on Friday and would remain there until the weekend.
Because power needed to be cut when a crane came to lift the train, work had to be done in the weekends. Spokeswoman Kimberley Brady said derailments were "not common" and the two recent ones were under completely different circumstances. "It is unfortunate."
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