Bulk of the network operates on overhead steel girders above road traffic. Overhead lines only use wood sleepers which are secured to steel girders. Wood sleepers create a damp contact with the steel girder thus reducing the life of steel and the sleeper. Customer also wanted a sleeper not to degrade like wood sleeper as maintenance and replacement costs are extremely high on the overhead network.
Customer migrated to using composite sleepers in year 2000 and replaced bulk of the track with composite.