I commend Ian's informative and encouraging post, and strongly disagree with LilLoaf's point of view. There is no doubt that experience in some instances is valuable, but to recruit solely based on a candidates' rail experience would be foolish, and I'm sure that's rare. Being 'in-house' is obviously an advantage, if you're looking at moving up in the same Company then you have direct access to HR staff, and it's a relatively easy process for the Employer as they know your 'form', but there has to be an element of ability and aptitude. Not all customer-facing staff would make good drivers, and not all drivers would make good customer-facing staff.
The same could be said for my industry - aviation, I don't have the characteristics or skills required to be cabin crew, so why would I become one in order to work my way up to becoming a pilot. Of course I've flown with some guys who are ex-cabin attendants, but in the main it simply doesn't work like that, and rightly so. Different skills, different jobs.