Consider spending a lot of time in math and science classes particularly in the physical sciences (Chemistry, Physics). Air brakes are a study in gas laws. You also should consider any shop classes that are offered. Strongly consider an engineering degree or other mechanical related advance degree. There are a lot of people who consult on steam engines who have engineering degrees. This way you can work in the shop when the locomotives are not running. It also gives you something to fall back on. Do listen to more experienced people when starting. You also need to think before doing something or you may injure yourself or be the latest victum of the running joke. Safety is always first. If you can, get involved with a preservation project in your area, this can open your eyes on a few issues.
A Road Foreman of Engines told me that he liked hiring mechanical people to run trains. He did not expect them to fix the problem if something broke, just desribe it intellegently. Comunicating intelligently impresses people. I am still learning how to spell, maybe one day. But for now concentrate on your education, that will take you far.