In message 01ca5720$d8b522c0$LocalHost@default, at 16:20:58 on Tue, 27
Oct 2009, Michael R N Dolbear quoted DfT thus:

In terms of their success rate, private prosecutions appear on the
whole to be highly effective in using the legal system to tackle fare
evasion.

Just as the police are not automatically the investigators of all crimes
[1], the CPS isn't the only prosecutor either.

It also used to be the case (not sure of current status) that "Car Tax"
evaders were also prosecuted privately, in that case by people employed
in regional DVLA offices.

[1] As a trivial but robust example, HMR&C investigates tax evasion.
--
Roland Perry