Carson Spur was a railroad spur that left the main line to San Francisco, California and in 1885 terminated at Shonash Ravine. After a bridge was completed over the ravine in 1886, the line looped back into Hill Valley near the Hilldale development, while crossing a few streets in town.
Doc Brown and Marty McFly planned to hijack a train heading towards San Francisco, decouple the cars from the locomotive at the beginning of the spur, and then push the DeLorean time machine sitting on the tracks to 88 mph before it went over the edge of the ravine. The DeLorean sat a half mile away from the junction of Carson Spur and the main line, across from an abandoned silver mine. Their plan was partially successful. After Doc and Marty realized that Clara Clayton had followed them and jumped on board the locomotive, Doc had to go back to save her from going down into the ravine. At that point while nearing 60 mph, they passed the Point of No Return, a windmill that indicated no turning back for the train or the DeLorean. Around a quarter mile to go, Marty sent the hoverboard he had gained from 2015 down the track where Doc caught it with his foot. Doc grabbed Clara before she fell to her death and they floated off into the distance.
Marty closed the door of the DeLorean and experienced temporal displacement once again. Back in 1985, the DeLorean came to rest on the tracks near Hilldale. At that moment a modern day diesel locomotive barreled towards the DeLorean and destroyed it just as Marty escaped, leaving a debris field across the tracks.
After Marty returned to the scene with Jennifer, the railroad crossing warning signals sounded and Doc's new train time machine appeared, blowing them away. Its entry point would have been the same line from either 1895 or 2015.
After some talking, Doc, Clara and their family lifted the train off the tracks and hovered. The train turned to build up speed and traveled to some unknown time above where the spur would have existed.