The official description
This memorandum summarizes a Central Intelligence Agency (CIA) debriefing of a group of four individuals who reported observing a “flying saucer” or “unconventional aircraft” in 1955. The group, which included U.S. Senator Richard Russell, a U.S. military service member, and two U.S. Government officials, reported observing a luminescent “greenish-yellow” phenomenon, as seen from aboard a train while traveling within the Soviet Union, in present-day Azerbaijan, between Baku and Tiflis (Tbilisi, Georgia).
Quoted from the U.S. Department of War, PURSUE UAP releases. Read every file for this incident in Official files or at war.gov/UFO.