The 503 Kingston Rd (temporarily replaced by shuttle buses as of fall 2016) is an east-west streetcar route in Toronto, Ontario, Canada, operated by the Toronto Transit Commission. The 503 Kingston Rd travels on a route to the downtown financial district from the Bingham Loop and operates in the rush hour only with a 12-minute service frequency. The route shares much of its track with the 501 Queen, 504 King and the 502 Downtowner.
The Kingston Road line only travels over a short section of Kingston Road and does not extend into Scarborough, except for about 100 feet as it travels north on Victoria Park Avenue before entering the Bingham Loop. Up to 1954, the Kingston Road line used to extend as far as the Birchmount Loop, on Birchmount Road.
More recently, as of fall 2016 the route has been operated using only supplementary buses due to a streetcar shortage. 
^James Bow (2013-08-24). "Remembering Birchmount Loop". Transit Toronto. Archived from the original on 2013-09-21. Retrieved 2014-02-18. The easternmost streetcar loop in the TTC's history is Birchmount Loop, located at the Kingston Road / Birchmount Avenue intersection inside the old City of Scarborough.