About noon on September 21, 1974, 2 loaded "jumbo" tank cars, cars 17 and 18 of a 145-car complement, were uncoupled as a unit at the crest of the gravity hump in the Southern Pacific Transportation Company's (SP) Englewood Yard at Houston, Texas. The two cars passed through the hump master retarder and group retarder without being slowed and accelerated as they moved down the grade into bowl track 1. At a speed of 18 to 20 mph, the two tank cars impacted an empty tank car. Upon impact, the coupler of the empty tank car rode over the coupler of car 17 and punctured the tank head. Butadiene spilled from the car and formed a vapor cloud, which dispersed over the area. After 2 to 3 minutes, the varpor exploded violently; as a result, 1 person died and 235 were injured. Total damages amounted to about $13 million, which included the destruction of 231 railroad cars substantial damage to 282 others.