Through sleeper renewals should be done when the percentage of unserviceable sleepers is equal to 30% on trunk routes/main lines and 35% on branch lines. Various conditions warrant the renewal of each type of sleeper, as explained in the following.
Wooden sleepers Wooden sleepers become unserviceable only when they are split, broken, burnt, or rounded on the underside with deeply cut rail seats. They are also considered unserviceable when the spikes that fix the rails on them lose their gripping power or when there is general deterioration in the structure of the sleepers as a whole due to an insect attack, etc.
Steel sleepers Steel sleepers become unserviceable when there is excessive corrosion and also when the rail seats or lugs crack or give way. The life of a steel sleeper is reckoned to be about 40 to 50 years.
CI sleepers CI (cast iron) sleepers become unserviceable when there is excessive corrosion of the tie bar and the rail seat, when the sleeper plate breaks or develops a crack, or when the wear of the lug and rail seat is so excessive that the sleeper no longer grips the rail firmly and the keys become loose resulting in creep. The average life of a CI sleeper is 40 to 50 years. Generally, these sleepers can be considered unserviceable when they are unable to either hold the gauge or retain the packing.
The renewal of worn out fish plates should be considered when the gap between the fishing planes of the rails and the fish plates exceeds 1.5 mm at the centre.