Metro drivers are being lured away by bigger salaries on offer at other rail operators.
Bosses responsible for the Tyne and Wear transport network have admitted they need to boost the numbers they train to keep the service running.
About a fifth of all delays every year are caused by ‘general operational issues’ - which includes the number of drivers available.
Chris Carson, Nexus’s metro services director told the North East Joint Transport Committee: “We have no problem attracting people to drive trains. We generally go out and recruit pools of candidates, which we then draw on about eight at a time.
“In recent years, in terms of retention, we’ve noticed that with increased timetables and what the likes of Northern Rail and freight operators are doing, and the salaries they can offer, it is more attractive for those drivers with the flexibility to move on.
“We’ve recently lost drivers to First Trans Pennine Express.”
According to a report prepared for the committee, a quarter of all delays over the summer were caused by a shortage of drivers.
Nexus hopes to improve reliability in the coming months by returning staff to driving who were ‘temporarily allocated to other duties’.
The operator is currently only able to train 24 new drivers a year, but hopes to be able to improve on that in preparation for the introduction of its planned new fleet of trains.
But even if they can manage to do that, bosses believe they will still be hamstrung by contracts which allow staff to quit within weeks of completing their training.
Mr Carson added: “We need to make sure the drivers we have are as operational as possible. We need to work with train crews to make their jobs even more attractive.
“We need to work out how to retain drivers.
“It takes five to six months to train a driver and we can lose them within four weeks.
“These are things we’re aware of and we think we know where to go to retain our drivers, but one thing we’re unlikely to do is match the very attractive salary on the national network.
First TransPennine Express declined a request for comment.
Northern Rail has been contacted for a response.
James Harrison , Local Democracy Reporting Service