I went through the selection process for LNER and Northern not that long ago. The assesment centre day was grueling but I was pretty chuffed at getting though for both companies.
You then sit in a talent pool until they run the next course.
Even if you pass the course, there’s no guarantee of a job at the end of it.
Think I dodged a bullet actually as the companies seemed disorganised. I was asked my notice period, which was 3 months. Few weeks later I was offered a start in 4 weeks. I had to decline. Then a month later, offered another starting in 3 weeks.