ONLY half of Sydney's busiest peak-hour trains run on time when using a tougher measure of train delays, a leaked internal analysis by RailCorp shows.

The documents show just 46 per cent of trains run within two minutes of their timetable on CityRail's Western Line in the busiest hour of the morning.

Failure to achieve better on-time running can be annoying for commuters. Credit:Tamara Voninski

The result is a far cry from RailCorp's published statistics showing more than 90 per cent of trains run on time, based on a more relaxed definition of trains arriving up to five minutes late.

The failure to achieve better on-time running can be annoying for commuters waiting for trains.