Premieres April 2024. They released the main PV here as well as an introduction video here. I am not familiar with this franchise, so I couldn’t really piece together how it is connected with other installments. To make up for that, here is an interesting bit of info I found while trying to figure it out. From an English article:
The Shinkalion (Shinkansen Henkei Robo Shinkalion) project, which features transforming bullet trains, was created by the East Japan Railway Company (JR East), Shogakukan-Shueisha Productions (ShoPro), and toy maker Takara Tomy. Its announcement in 2015 was preceded by the display of a concept model of an E5 train mecha at Tokyo Toy Show 2014 by JR East. The Shinkalion toys are part of Takara Tomy’s Pla Rail series.
You never know what clever writers can do with a seemingly limited premise. I always take something like Gold Lightan’s cigarette lighter shaped transforming robots and the story that turned out to be quite good as a reason for optimism.