I have no empirical evidence but my guess is that for railway offences it's more often used by the prosecution than the defence. I think this would be so because it would likely take long enough for the defence to request it that it would have already been deleted.Yes, there are many posters on here willing to take matters to court if it were them. Often quoting CCTV evidence. Is CCTV actually used as a defence very often?
Most organisations keep CCTV for around 30 days - how often do people receive their notice of intent to prosecute before then?