Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
There were many books about trains. Below were some classic recommendations:
Strangers on the Railroad by Camo Bryant
2. A Single Person's Train Travel-David Pennilson
3. The Train and the Girl-Raymond Chander
4 The Story of the Train-Robert Dieber
A Train Journey-John Emsley
These books cover different aspects of train travel, including adventure, romance, suspense, history, etc. The readers can choose to read according to their own interests.
It's hard to pinpoint exactly where Gojo is at any given time in the manga. His whereabouts change as the story progresses and different scenarios unfold. It could be in a supernatural realm, a mission site, or interacting with other characters.