The Cottingham Road bridge in Corby was originally due to be finished in mid-summer.
Local people had agreed to put up with the disruption as it was only due to last just over six months.
But the Network Rail scheme has since been hit with several delays that have lefts locals in despair.
Earlier this mohth, the Northants Telegraph launched Bridge Watch to put pressure on Network Rail to ensure that this time, the bridge linking old Corby with new Corby, reopened when they said it would on October 19.
At a public meeting ten days ago, residents heard what exactly had gone wrong.
Project sponsor Wendy Bell said: “The bridge was manufactured in Ireland. Some components were manufactured off-site and brought to site.
“There was a trial erection in Ireland to see if they could fit the bridge in 30 hours, which was the amount of time we would have in Corby because we’d need to close the railway line
“That went incredibly well.”
More than 15 tonnes of steelwork also went into the bridge and Amco, Network Rail’s contractors, had to work with several different utilities companies whose cables ran over and under the bridge so that supply was not interrupted while the bridge was taken out and replaced.
Wendy added: “In April we thought we were in a really good place. We were comfortable with the works and still on target to deliver.”
But when some components arrived on site, Amco contractors found there was a problem. A crucial set of units were 40mm out.
Wendy added: “ Between design and manufacture there was a mistake.
“But we were confident we could sort that out.”
The mistake knocked the project back by two weeks.
However, then workers found that the footpath leading on to the bridge was not stable enough, and may collapse. Rather than helping accelerate the works, the hot summer had made the ground around the path crumbly so Amco had to do significant work to shore up the whole length of it.
Utilities companies were then concerned that, after the additional works, there would not be sufficient depth to lay their cables.
Talks between the companies then delayed progress even further.
The bridge is now due to partially reopen on October 19.
Network Rail have already launched a full investigation into what went wrong.