Final answer:
The Great Locomotive Chase, also known as Andrews' Raid, is the event that happened in Big Shanty, where Union troops led by a Confederate spy attempted to sabotage Confederate rail lines.
Step-by-step explanation:
The important event that began in Big Shanty, Georgia involving Union troops and a Confederate spy working for the Union is known as the Great Locomotive Chase or Andrews' Raid. It occurred on April 12, 1862, when Union raiders led by James J. Andrews stole a locomotive named 'The General' with the goal of sabotaging the Western & Atlantic Railroad between Atlanta and Chattanooga.
Despite their efforts, the raiders were pursued and eventually captured. The outcome of this event was not in favor of the Union; the raiders were caught, and some were executed as spies, including Andrews himself. It showcased both the daring of Union espionage and the resilience of the Confederate forces against Union sabotage efforts.