 Originally a rush-hour service, the route was upgraded in September 2019 to run weekdays excluding evenings after the consolidation of 502 Downtowner service into this route.Starting at the Bingham Loop on Victoria Park Avenue, the 503 runs southwest on Kingston Road then turns west along Queen Street.At the Queen Street Viaduct bridging the Don River, it then turns southwest along King Street, enters the King Street Transit Priority Corridor and continues to Spadina Avenue, where it turns on the Charlotte Loop (via Adelaide Street West and Charlotte Street).