Also note that this is only for track bikes with a perfectly straight chainline (technically speaking it would also work for internal gear hubs wish are increasingly driven by belts in the upper price range). The shifting chain that is deployed diagonally more often than straight is an entirely different beast, in theory much worse in efficiency and durability. Pretty much all practical durability limits stem from compromises done for that diagonal capability for sitting. Which basically goes ruthlessly against all mechanical engineering rules, like the old adage about how bumblebees were supposedly unable to fly. Yet even they work quite well, given that challenging background they work just awesome and track chains already work better than that.