Class I railroads hiring and promotion procedure is usually completely different from shortlines. I will just talk of Class I BNSF for now. You asked about a lot of different questions and scenarios that don't have an easy one answer.

Hiring out on the BNSF you would expect to be trained as a Conductor, Brakeman, Switchman, and Hostler. Which one of those positions you will work will be decided on your seniority and desire.

Opportunity for promotion to engineer will happen based off of seniority. Whenever the company deems it necessary to promote more engineers they will ask for internal applicants in the seniority district and pick based off of seniority of those who apply.

As for a move into Trainmaster or other management position, you can start applying after your probation period. You will be hired based off of your experience and skills. Further promotions in management will be based off of performance. Trainmaster and most other management positions can be open to non-railroaders too. Typically people hired from outside the rail industry seem to be preferred on class I railroads. Their decisions will be based off of what they were taught and not what they learned from working in craft. Basically that means they won't decided based on what will be best for the train crew, decisions will be made on what is best for the company, the crew will have to deal with it.

As for how the industry is, there are A LOT of guys furlowed right now and I don't see Class I hiring to happen for some time.

As for college, BNSF will pay for some or all (I don't remember) of your courses if they apply to your job. They have to be done during times that will not effect your job on the railroad though. If you have to miss work to attend college classes you will loose your job on the railroad.