For two decades, one of the most commonly used type of rail tanker has been allowed to haul hazardous liquids from coast to coast even though transportation officials were aware of a dangerous design flaw that almost guarantees the car will tear open in an accident, potentially spilling cargo that could catch fire, explode or contaminate the environment. The rail and chemical industries have committed to a safer design for new tankers but are pressing regulators not to require modifications to tens of thousands of existing cars, despite a spike in the number of accidents as more tankers are put into service to accommodate soaring demand for ethanol, the highly flammable corn-based fuel usually transported by rail.