Rail maintenance jobs in Sydney are under threat from new trains replacing NSW’s ageing regional fleet over the next few years, leaked government documents show.

And they reveal that the state's transport agency has identified "industrial relations" as a key issue in the project to replace NSW's decades-old fleet of XPT, Xplorer and Endeavour trains.

"Once the existing regional fleet is retired, a number of Sydney Trains employees involved in the maintenance of the existing regional fleet are expected to be displaced," the internal Transport for NSW documents state.

The state's XPT trains are among those due to be retired by 2023.

"A comprehensive transition and industrial relations plan will be established to manage potential redundancies and other IR issues."