The operating service is a bit illegal, let's say everyone knows but everyone also knows that no one knows.
For the touristic trains you indeed have to get a license: you need to be medical checked up but also have to be mentally tested.
When you've got that you will have a license that states that you are allowed to be trained to a traindriver on touristic trainlines.
The training itself for a certain locomotive type can vary between 1 year (shunting) to 4 years (steamengine)