Good question. The composer and artist of most of them in Japan is Minoru Mukaiya.[1] He's also the CEO of Ongakukan, which builds train simulators for both games and training.
He's done over a hundred original station jingles.[2] Many of the Yamanote Line jingles are classics, though.
[1] https://en.wikipedia.org/wiki/Minoru_Mukaiya
[2] https://www.ongakukan.co.jp/en/business/music/#melody