Varies a lot location by location.
Many do nights on the depot 2200-0600, some do day shifts and work as travelling or turnaround cleaners, all depends on the depot. Those that do nights often get away early at many locations once they have cleaned their quota of allocated trains; Newton Heath, Manchester had a reputation of being deserted by 2am, all that has changed nowadays I believe, but not by much.
It's a not a great job, but does get people that crucial first step onto the railway, and offers full travel passes, unlike other rubbishy train companies who contract staff out to Carlyle, ISS et al.
