As
@Stigy says, you'll get a licence, but they're similar in car licences, in that they say what / where you can drive on the digital copy.
Depending on location, depends on what you do rules wise. The 'junior driver' role is relatively new in the scheme of things, so things are always changing.
In some areas, previously, they were only allowed to shunt in certain parts of the yard and couldn't move engines or trains from the depot to other yards nearby. Their rules were done to mainline standard and they can now do this.
I am a qualified driver.
You'll generally be moved up to mainline when you're needed, which, depending on the area, can be quicker than others. Or if you're wanting to go elsewhere in the company, on the needs of that area. E.G. if you lived in Doncaster and were applying for junior in South Yorkshire, did a year or so there and a vacancy came up at West Yorkshire and you wanted to go there, as long as they could facilitate the training, that may be possible.
It's a good job, good entry into the role. People should take it for what it is, experience, and not be thinking they're 'owed' a mainline job (not aimed at you or anyone in particular, but there's been a few of these sorts before).
It'd usually be shunting engines and/ or wagons from one side to the other.