A 4-car DMU was travelling from St. Pancras to Luton when a fire broke out under the rear power car (M51594) about 1 mile north of St. Albans. The communication cord was pulled at the same time as fire alarm bells rang in both the driver's and the guard's compartments. After the train came to a stand, the driver went back to attack the fire and the guard detrained the passengers without injury. The fire remained confined to the last vehicle which was extensively damaged as the fire penetrated the floor of the passenger compartment in four places (the vehicle was subsequently deemed as beyond repair).
The fire was caused by a small leak in the exhaust (failure of corrugated metal gasket in a bolted joint) which was directed at the main fuel tank. Spilt diesel oil fuel which had been absorbed by the felt packing between the fuel tank and its supports then ignited, fueled by the oily deposit of dirt over the surface of the fuel tank. Once the fire reached the top of the tank it ignited a pool of fuel oil which had collected behind a raised lip on the top of the tank and then ignited a jet of vaporised fuel oil via an open breather vent pipe. A flashover into the inside of the second fuel tank caused an explosion which split the upper seam of the tank, resulting in intense jets of flame being directed at the underside of the floor. The entire fleet of power cars subsequently received replacement fuel tanks with flush tops and the felt packing strips between the tanks and their supports were removed. Modifications to the routeing of both the breather vent pipe and the oil return pipe from the torque convert were also carried out to prevent another similar fire. Additional modifications being considered at the time included re-routeing the exhaust pipe from the No. 2 engine to avoid passing near the main fuel tank.
MOT; 8pp; (I.K.A. McNaughton).