You can swap a shift with someone else at your depot for one day providing you both have 12 hours rest between shifts.
You can swap a rest day, but again you both need 12 hours rest before any shift.
A further complication which isn't the case at the moment, but will be in the future, is that both of you will need to sign the same routes and traction. There will be different 'links' (The railway term for rotas) at some depots which will have different work content in this respect.
Some of your leave will be Annual Leave Weeks (allocated to you and rotate around each year, which you can swap mutually with other drivers), and the rest will be Annual Leave Days. There is a quota of so many Annual Leave Days which will be guarenteed to be granted each day at each depot. Any beyond that are 'pending' and you'll only get the day off then if there is someone who can be allocated off 'spare' to it, or if there is someone willing to work their rest day.