One other point very much worth considering, which is, clearly, only my humble opinion, is that many of those who choose to take non-driving grades with a view to becoming a driver through the internal route, quickly become their own worst enemy when the hoped-for opportunity doesn't come along quickly enough.
The result of this appears to me to be that people who are in a role (typically that of conductor) which has pretty much a completely different skill set than that which is required for driving, also quickly gain an attitude that can be seen from miles away - "External candidates got the job, so why can't I? I'm a victim." This is a double whammy. A large number of aspiring drivers do not seem to know that there is almost NO overlap between a customer facing job and a driving job - safety critical or otherwise: however, recruiters do know this. It is only the peripheral skills such as appearance and reliability that are transferable, so if the person has allowed an attitude to develop, then overall, this is more negative than positive.
I see this time and again and it stands out like a black cloud over that person's head. I really really think that many people would have been better served by applying directly for the job they wanted, when it came up, and not getting embroiled and perhaps trapped in a second or third best role (in their minds) - albeit a valuable and crucial, relatively well-paid role in many cases.
Slight ramble, but something which isn't raised enough, I believe. Absolutely no offence intended to anyone - just an observation from the footplate which I hope will assist applicants' decision making.
Regards,
R