I'm not sure I agree about the promotion. The trade is about 30 strong at best and they have one WO1 position and at least one WO2 position. I understand the pyramid is a reasonable shape and that able candidates can progress as well as in any trade. When a Railwayman joins up he has a one in thirty chance of making WO1, much better odds than being a Rad Op.
Promotion to LCpl and Cpl is, on average, slow across most of the driver related trades. However, for those who make WO1 before their twenty year point, they have been promoted about every third year, much like half of the trades in the Corps, surely.
You wouldnt walk straight into a driving Job with a TOC, but it would give you a good head start over other civvys applying for a overpaid seat warmers, sorry I meant train drivers job, the quals are different.
Regards to a transfer, it used to be something like, transfer and keep your rank, but you must reach relevant qual for rank within a specified time or you lose you rank, I dunno, someone else will put you right!