Hi Harry!
Welcome to this forum!
I'm pleased to hear that you have found many interesting posts over the last few days. If you continue your application (and I hope that you will), you will find many of the answers to your questions already here if you use the search facility. If you have any further questions, please do not hesitate to ask.
TOCs and FOCs take trainees from both internal and external routes. Being a shunter could be a step towards being a train driver (especially with a FOC); in the case of a TOC, a guard (aka conductor, train manager) would probably be a more worthwhile step. However, there's nothing to stop you also applying straight away for trainee driver without the stepping stone.
Attempting to be a train driver is always a long shot, with reports of between 300 to over 1,000 applicants per job, and with the covid-19 situation, these ratios are not going to improve. However, I knew that it was a long shot when I applied and was also planning for plan B, but some people have to be successful, and there's no reason why it can't be you! Even though you would be an external applicant, 17 years of experience with Network Rail should (I would imagine) work heavily in your favour!
As your job is secure with Network Rail, hopefully you can apply for positions while keeping your current job until you have an offer.
A few tips:
* Visit the jobs/careers portal of all TOCs and FOCs in your area. Many will allow you to subscribe to new job ads so you will get an e-mail when new jobs are posted. Nonetheless, check the webistes of each of the TOCs and FOCs in your area every day for new vacancies.
* Subscribe to the
https://www.railforums.co.uk/forums/railway-jobs-careers.122/ forum so you get an e-mail for every new thread. Sometimes users post jobs here, and you will also find other valuable information.
* When you see a vacancy, be aware that they might be taken down when a certain number of applicants apply, even if before the deadline (I've known one taken down within 18 hours!). That's why it's important to check the websites of potential TOCs and FOCs every day. Most will require that you answer some questions (essay-style). Rather than rush these before the ad is taken down, you would be well advised to prepare some in advance and then just adjust pre-prepared answers to the questions. You are almost certainly going to be asked why you are applying for the role, and what knowledge, skills and experiences you have which qualify you for the role. They might ask questions where they are trying to assess your commitment to safety, your customer service experience, why you are leaving your current role and/or what you do in your personal life. When preparing your answers for such questions, look at job descriptions for the role and try to include as much as those in your answers as you can. When you revise your pre-prepared answers for a vacancy, again, ensure that you have addressed their job description and what they're looking for in your answer.
* Familiarise yourself with the rest of the process, including assessments. The Group Bourden Test is what you'll want to practise the most, as well as thinking ahead to the MMI questions!
I really hope that you do make it to trainee train driver, whether or not you make a stepping stone (shunter, guard, etc.) on the way there. The recruitment process is lengthy - don't expect any quick answers, but it will hopefully mean those who are just applying because it's advertised then rather than because it's a true goal will drop out. It will be gruelling and stressful - it's part of how they choose. But your experience with Network Rail will, I believe, stand you in good stead!
Once again, welcome to the forum. Search then ask if you need any more information!