Brixton is perfect station for free riders into London. Close to central London by bus (or tube) but no barriers. You can come all the way from zone 5 or 6 and only pay single bus or tube fare. Madness. Fare evasion is rife on this line, only Orpington and Bromley South have gates.
Brixton is also absolute mess of a station...
Its also quite funny at Brixton watching the chancers from Petts Wood, Bickley, Beckenham etc who jump out quickly to touch in on the readers on the up platform and then get straight back on the same train to Victoria. Its laughable. Nunhead is another zone 2 station where this practice is rife. They'd only have to move the readers downstairs which would be easy at Nunhead but presumably at Brixton they can't as the stairs lead directly to a public street?