A couple of days ago, I needed to go to Croydon, whither people usually go when there is a need, such as going to the shops, and I very much doubt it is somewhere people visit to enjoy the sights and the ambience.
Being stingy, I wondered what would be the cheapest way to reach Croydon from Southfields, the nearest tube station for me. I had initially thought I should go to Wimbledon on the District line, and take the tram to Croydon, but I realized that the tram operates on a separate fare system from the underground, i.e. there is no through fare, and as such I would be paying for a single fare from Southfields to Wimbledon (within zone 3: :1.60 peak; :1.50 off-peak), and another single fare on the tram (:1.40), making an off-peak journey :2.90 on Oyster pay-as-you-go.
Thinking that there must be a cheaper way, I looked up TfL’s Single fare finder, which does not include the tram system, and found that a single from Southfields to East Croydon costs :3.20 peak and :2.20 off-peak, and to West Croydon costs :2.50 peak and :1.90 off-peak. Going to West Croydon by rail is cheaper than taking the tram by one whole pound during the off-peak period.
East and West Croydon stations are not far apart, both are in zone 5, and either would do for my purpose, so naturally I decided to go to West Croydon. My guess is that the route calculated for the fare for East Croydon involves travelling from Wimbledon to Clapham Junction (zone 2), change trains there, therefore the zone 2~5 National Rail fare applies, whereas West Croydon can be reached via Sutton, without venturing into zone 2, so the zone 3~5 National Rail fare is applicable. However, as services to Sutton from Wimbledon run every half hour during most of the day, but services between Wimbledon and Clapham Junction, and between Clapham Junction and West Croydon are more frequent, I travelled to Clapham Junction, then on to a train bound for Sutton that took me to West Croydon.
It was obviously rather silly on my part, since taking the train instead of the tram took me almost as twice as long to reach Croydon, for a saving of a pound, and I should value my time more, however I always find these kinds of quirks in the system rather amusing.