CITYRAIL commuters have been promised some respite from rapidly worsening crowding on peak-hour trains - in about 18 months.

The rail operator's latest snapshot of peak-hour crowding shows a sharp deterioration in the conditions faced by morning and afternoon commuters.

In the year since March last year, crowding levels on morning peak services increased from an average 119 per cent to 123 per cent.

CityRail measures 100 per cent crowding when all seats are taken. At 135 per cent the train is full enough that it struggles to run to timetable because of delays in loading and unloading passengers.

But on two lines, the Bankstown and the Northern line, morning crowding levels are at an average 150 per cent.