The Fandom Post is reporting that Pluto TV has added the four-part The Galaxy Railways: A Letter from the Abandoned Planet OVA series to its service.
The OVA was a bridge between the two seasons that were created, even though it came out in the middle of the second season. The first series was licensed and released by FUNimation Entertainment in the United States but they did not pick up any further rights after that as it did not perform well enough to merit it. Discotek Media acquired the license for the OVA in 2015.
Source: The Fandom Post