Whilst it's technically the wrong way round...
2R01 0456 Balham - London Victoria was once just in passenger service from Clapham Junction, IIRC. I know there certainly used to be a Clapham Junction - London Victoria service around that time, as recently as about 3 or 4 months ago, but that appears to have disappeared, and seems to start back at Balham now. I think the headcode and timings are the same from Clapham Jn. The logic was that the stock (5R01) could be sent from Selhurst Depot via Norwood Junction or Selhurst and so it was only practical to start it at one or the other, rather than further towards Croydon. Likewise its staff stop at Battersea Park has never been advertised to enable it to use the Battersea Reversible avoider instead (where crew knowledge permits, which isn't exactly always the case nowadays...).