 To provide a reasonable service on the North Shore line, trains were transposed at Central, resulting in delays to trains and causing trains to catch up to those in front.A down electric commuter train "tripped" past a signal showing stop at the down end of Roseville platform into the section occupied by the preceding train.After recharging the brake air supply, the driver proceeded at above a cautious speed (40 mph rather than 15 mph) and came into violent collision with the train ahead.