(Chief Inspecting Officer of Railways, Department for Transport), in the Sir Seymour Biscoe Tritton Lecture to the Institution of Mechanical Engineers in 1977. Speaking of the fall in fatalities since 1955, he put forward his opinion that a major factor in this improvement was "the introduction in 1951 of the BR standard Mark 1 passenger carriage, which, over a period of about ten years, replaced pre-war designed rolling stock on most principal routes. The damage-resistant qualities of this all-steel coach, mounted on a 200 ton end-load resistant underframe and fitted with buckeye couplings, have been evidenced time and time again. Only in a small number of very destructive accidents has serious body damage of the kind that inevitably leads to fatal accidents been observed and there have been several remarkable instances of high-speed derailments in which no personal injuries have occurred." Although construction of Mark 1 passenger stock ended in 1963, multiple units and non-passenger carrying stock based on the Mark 1 design continued to be built until 1974. The 1988 Clapham Junction rail accident highlighted that by the 1980s the Mark 1 coach was dated, and less able to withstand collisions than newer designs. Nevertheless, the Hidden Report into the disaster concluded that withdrawal of Mark 1 units was impractical and the design was not inherently unsafe: "The inventory of Mark I coaching stock is large, and much of it has not reached an end of economic life, nor will do so for another decade or more. Mark I vehicles have good riding qualities, and are not intrinsically lacking in collision resistance." British Rail was still using some 4EPB and 2EPB (classes 415 and 416) multiple units with underframes that had been constructed before World War II and these were a priority for replacement. During the late 1990s Mark 1 stock began to reach the end of its design life and concerns about its safety relative to newer rolling stock became more pressing. The Health and Safety Executive issued instructions in 1999 per the Railway Safety Regulations 1999 to withdraw all Mark 1 carriages and multiple units based on that design by the end of 2002 unless rebodied or modified to reduce the potential for overriding in the event of a collision. A proposed modification to extend mainline use beyond 2002 at the time of the 1999 HSE instruction was 'cup and cone', however trials were inconclusive and deemed