Wakana Kingyo

A mysterious bullet train foretells giant monster attacks - the Black Shinkansen! One day, grade schooler and train fan Hayato Hayasugi discovers his father's secret - he runs the Shinkansen Ultra Evolution Institute, which has created the defense against monster attacks - bullet trains that turn into giant robots, Shinkalion! Hayato becomes a Shinkalion driver and joins kids from across Japan to fight against the threat of the Kitoralzers! (Ununnilium)

Half a year after the defeat of the Kitoralzers, mysterious giant beings suddenly appear from space. The Ultra Evolution Institute has been busy developing a new prototype unit - the Shinkalion ALFA-X. Suddenly, a 9-year old Hokuto appears from across time and space. Having no choice, the Ultra Evolution Institute appoints the young Hokuto to become the driver of the Shinkalion ALFA-X, bringing in the teams from across Japan as the last line of defense against the invaders. (Wikipedia)