|Vehicle||BYD electric bus / Alexander Dennis Enviro 200|
|Peak vehicle requirement||67|
St Paul's station
|End||London Bridge station|
|Frequency||Up to every 2 minutes|
|Operates||Mon-Fri (Except public holidays)|
London Buses route 521 is a Transport for London contracted bus route in London, England. Running between Waterloo and London Bridge stations, it is operated by London General and is one of two Red Arrow branded services. In 2016, it became the first battery electric bus route in London, along with route 507.
See also: Red Arrow (bus)
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.
On 2 June 2002, along with route 507, the route was the first bus route in London to be converted to articulated bus with Mercedes-Benz O530G Citaros.
During late 2003, early 2004, a series of onboard fires on Mercedes-Benz O530Gs led to withdrawal of the entire fleet, while Mercedes-Benz made some modifications. During this period limited services operated using a variety of different buses on route 521.
In September 2009, as part of the move to replace London's articulated buses, the O530Gs were replaced by Mercedes-Benz O530 Citaros.
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.
In December 2013, two trial BYD electric buses were introduced. In September 2016, Alexander Dennis Enviro 200 bodied BYD electric buses began to operate the route, making it and route 521 the first battery electric bus routes in London. In September 2016, the buses on this route and on the 507 have received new digital route displays.
Route 521 operates via these primary locations: