While vigorously having sex your hog pops all the way out and then catches the rim on way back in, causing it to bend in half sideways like the boxcars in a train wreck. A bad case of liquor dick will increase the chances of this occurring.
1. "Man, the other night she was riding me when I had a huge train wreck. My dick has been blue for two days."
2. "I was banging her from behind last night but I was so hammered I kept having train wrecks."