Excessive wear in the torque convertors of the Class 127 units was being investigated by Rolls-Royce, and M51622 was fitted with various transducers along with measuring equipment located in the Guard's Compartment. One of the transducers was a strain gauge weighing 28lbs was fitted to the middle of the No.2 cardan shaft. The shaft carried the weight of the strain gauge which was fitted with restrainers to stop it rotating with the shaft.
Unfortunately, no-one gave a thought as to the stresses inflicted onto the shaft. It was later discovered that the Hardy Spicer 1800 shaft had a 'whirling' speed - the speed at which the shaft would bow (bend) and break-up - of 3100rpm, equivalent to a rail speed of 97mph. The additional weight of the transducer reduced this to around 67 mph. The unit was placed into service and formed the 08:07 Harpenden to St. Pancras with M51622 being the fourth car with another unit to the rear. At 08:19 the shaft failed, rupturing the fuel tank and debris caused rupturing of the fuel tank on the following M51632. The train was engulfed in flames and smoke and 33 passengers required hospital treatment. Fortunately, the train came to a stand adjacent to Handley Page's airfield at Napsbury. Their chief test pilot had seen the train approaching, on fire, and called out the airfield fire tender which controlled the blaze until the arrival of the Fire Brigade.