A major upgrade to NSW's decades-old fleet of Tangara passenger trains – already running eight months' late – is unlikely to be finished until next year because of challenges installing new technology.

The overhaul is aimed at extending the working life of the Tangara fleet by about a decade by improving their reliability and safety, helping Sydney's rail network cope with a forecast 21 per cent surge in passenger trips over the next three years.

Sydney Trains chief executive Howard Collins says crucial parts of the upgrade will be prioritised. Credit:Kate Geraghty

Asked why the $219 million project was running late, Transport Minister Andrew Constance said that "we are dealing with very old trains and we’re fitting new technology to them".

Mr Constance said Sydney Trains and the state's main transport agency would pressure the contractor to deliver the project "as quickly as possible".