As some people said, some roles are advertised internally only. All roles must be advertised internally.
Being an internal candidate will almost certainly assist you getting through the sift, and that can be half the battle!
Being an internal candidate won't directly help you with assessments, although you might know more people who can help you prepare for these - but you've got us for that anyway!

It's debatable whether it would help you in a DMI, but I would say that experience of the railways and the railway way of doing things, policies, routes, etc. would give you another advantage here.
In summary, I'd say that internals probably have a huge advantage!