No details are known, but it could be anything from some promotional tie-in with a train service, to possibly a reissue or new version of the combiner.
I'm personally hoping for a new version of the combiner (either like the Voyager & Deluxe combiners or like the Titan sized combiners). A reissue would be great too.
I am hoping it's not simply some promotional event / tie-in with a rail service.
https://twitter.com/TF_pr/status/1185042288549875712
12/3(火)~ 全国の鉄道会社全面協力で実施される「特別展 天空ノ鉄道物語」にて、#トランスフォーマー ザ☆ヘッドマスターズで、6体の電車から合体する人気キャラ「ライデン」の当時品、そして新プロジェクトの展示が決定!
詳しくは↓↓
https://tentetsuten.com
Leiden project started! 12/3 (Tuesday)-"Special Exhibition Tenku no Railroad Story" will be held with full cooperation of railway companies nationwide. #トランスフォーマー The ☆ Headmasters has decided to display a new project of the popular character “Leiden” that combines from six trains and a new project! For more information ↓↓https://tentetsuten.com
Japanese fans are hoping for either a new version or a re-release / reissue of the original.