the.com/runaway train

means A train that has lost control of its brakes or crew and is speeding on its own, unstoppable until it crashes or runs out of track — used figuratively for any situation, emotion, or process racing beyond anyone's control.

from Straightforward compound of 'run away' — to flee or go out of control — bolted onto 'train' once railways became common enough for the image to land. The literal sense (a train rolling without a functioning engineer or brakes) is a 19th-century railroad-age fear made literal; the figurative sense, applied to runaway inflation, runaway emotions, runaway thoughts, followed naturally once the picture of an unstoppable, out-of-control machine was fixed in the popular imagination.