"The Galaxy Railways: A Letter from the Abandoned Planet" is a two-episode OVA that serves as a sequel to the original "The Galaxy Railways" series. The story is set in a futuristic universe where interstellar travel is facilitated by a vast network of space railways. The Space Defense Force (SDF), a military organization responsible for maintaining order and safety across the galaxy, operates these railways. The narrative centers on the Sirius Platoon, a unit within the SDF, and their mission to investigate a mysterious distress signal originating from a long-abandoned planet.

The distress signal is traced to Planet Maetel, a world that was evacuated decades ago due to environmental collapse. The Sirius Platoon, led by Captain Yuuki, is dispatched to investigate. Among the crew is Manabu Yuuki, a young and determined officer who is the younger brother of the late Mamoru Yuuki, a legendary SDF officer. Manabu is driven by a desire to honor his brother's legacy and uphold the values of the SDF. The team also includes Louise Fortner, a skilled and composed officer, and Bruce J. Speed, a veteran with a no-nonsense attitude.

Upon arriving at Planet Maetel, the team discovers that the distress signal was sent by a group of survivors who have been living in isolation for years. These survivors, led by a man named Dr. Daigo, reveal that they have been conducting secret experiments to restore the planet's environment. However, their efforts have inadvertently awakened a dormant alien entity that now threatens their survival. The entity, a massive and powerful being, begins to attack the survivors and the SDF team.

As the situation escalates, the Sirius Platoon must devise a strategy to neutralize the alien threat while ensuring the safety of the survivors. Manabu takes a central role in the mission, demonstrating his growth as an officer and his commitment to the SDF's principles. The team faces numerous challenges, including the alien entity's overwhelming strength and the harsh conditions of the abandoned planet. The story explores themes of duty, sacrifice, and the resilience of the human spirit in the face of adversity.

The OVA concludes with the Sirius Platoon successfully defeating the alien entity and rescuing the survivors. The team reflects on the mission's outcome, emphasizing the importance of their work in protecting the galaxy and upholding the ideals of the SDF. The narrative leaves room for further exploration of the characters' journeys and the broader universe of "The Galaxy Railways."