The total rolling friction of a train on straight level track. It is generally taken as 30 lb per short ton weight (15 kg/t) of the train for cars having plain bearings and 20 lb/st (10 kg/t) for cars with roller bearings. These figures may be increased by 10 to 15 lb (5.0 to 7.5 kg) if the track is in poor condition and may be less for track in excellent condition.