Pros
Lots of locations to work across the UK, salary is about industry average, excellent graduate induction, very easy to stand out as hard-working or innovative. Lots of extracurricular programmes to lead. You will always have a job at Network Rail. A piece of advice I have lived by is, If you can't see yourself in senior management, then find a company you can. No point in doing something you aren't passionate about.
Kontras
Low-level work, stuck in victorian mindset (i.e. failing to adopt new technologies to increase rail performance and reduce costs over the long term), reactive rather than proactive industry, slightly dodgy in how commercial contracts are handed out, some nepotism. If you get accepted, talk to seniors who have been ex-grads and gauge if they still have excitement for the job. from this, you can decide if it is a good long term career.