Prisoners may spend the latter part of their sentence in open conditions where (under licence) they are able to come and go from the prison somewhat - home for short breaks to reestablish family relationships, for instance, or out to work placements to practise for holding down a job, or even to the supermarket to get used to budgeting and shopping again.
For some trips they may be taken by prison transport and for others they will use public transport. To facilitate the use of public transport they are given a travel warrant, a voucher that they can pay with. Of course this has the unfortunate effect of making them stand out a bit and railway or bus service staff may be curious about them. Some prisoners are quite blase about their status and don't care who knows while others are more self-conscious.
A prisoner told me yesterday that he'd been on a bus the other day and the driver, when presented with the warrant, said, "Oh are you a prisoner. What are you in for?" to which he immediately gave the reply, "I'm in for murdering nosey bus drivers"! What a good answer. :)
(P.S. This is not a photo of the bus driver in question of course. It's the former "Spencer" of the kids television show Balamory. The actor, Rodd Christiansen, became a bus driver in Scotland for a bit after the series was finished, until moving back to his native USA. Before Balamory, Rodd did some work with Scripture Union Scotland and my husband worked alongside him a couple of times. So when I was thinking a picture of a bus driver would be an appropriate accompaniment to this wee post I knew whose name to enter on Google...)