[1] Route 521 commenced operating on 18 July 1992 as part of the Red Arrow network of bus routes aimed at commuters in Central London linking some of the capital's main railway termini.

During this period limited services operated using a variety of different buses on route 521.

[5] Another criticism of articulated buses was the low number of seats, with only 49 per vehicle.

A standard rigid Citaro has 44 seats, however the new ones for route 521 have just 21, with room supposedly for up to 76 standers, leading to criticism the new buses were "cattle trucks" and even more crowded than the buses they replaced.

[6] In December 2013, two trial BYD electric buses were introduced.