About 4:00 a.m. mountain standard time on April 3, 1983, a Denver and Rio Grande Western Railroad Company (D&RGW) switch crew was switching 17 cars in the D&RGW's North Yard at Denver, Colorado, when a coupler broke on the 4th car, leading to an undetected separation of 150 feet between the 3rd and 4th cars. The engineer, responding to a hand lamp signal from the foreman, accelerated the locomotive, with a caboose, an empty freight car, and a loaded tank car coupled ahead. The loaded tank car impacted a fourth car at a speed of about 10-12 mph. Upon impact, the end sill of the fourth car (empty boxcar) rode over the coupler of the (loaded tank car) and punctured the tank head. Nitric acid spilled from the car and formed a vapor cloud which dispersed over the area. As a result, 9,000 persons were evacuated from the area; 34 were injured. Damage to railroad property was estimated to be about $341,000.