Ding-Ding! That old-fashioned red double-decker bus, the one that has graced scores of movies about London and become an international symbol of Britain, is tooling off into the Waterloo sunset.Along with twice-a-day mail service, the red telephone box and (gads!) fox hunting, it is the latest symbol of old England to come under assault in the age of Cool Britannia. The transport authority, which incidentally is led by an American expert in transit efficiency, says that sentiment aside, the Routemaster is costly because it requires both a conductor and a driver; passengers are forever injuring themselves when they alight from its open back platform while the bus is still moving; and its narrow aisles and steps are a nightmare for people with wheelchairs or baby strollers.Logical reasons all, but there is no accounting for love. A small army of Routemaster lovers has mounted a vigorous campaign to try to keep the buses going beyond their planned retirement next year. Their efforts look doomed to fail, although Transport for London has conceded that it will retain a few ‘‘heritage’’ routes in the middle of the city — a sort of rolling museum to let the tourists know what a real London double-decker was like. (Modern, boxy double-deckers will remain, but they just aren’t the same.) ‘‘Disgusting,’’ said Charlie Scott, general manager of the charter company Blue Triangle Buses and a former Routemaster driver.The RMs — as aficionados know them — have driven their way into the hearts of Londoners for many reasons. They have been a fixture on the streets since the 1950s; so most people have grown up with them. Their curvy, aerodynamic design is as comforting as an old Electrolux refrigerator, with rivets that recall World War II airplanes.The RM was built tall and narrow to negotiate city streets, and the upper deck is so low-ceilinged that a 6-foot-tall commuter must stoop to stand.The Routemaster was conceived in 1947 to succeed London’s high-capacity trolley-buses. Its designers drew on wartime aircraft manufacturing techniques, creating an aluminum body that was light and flexible — this in an era before aluminum was commonly used in vehicles.Some 2,760 Routemasters were put into service between 1956 and 1968; up until the 1990s, 1,000 remained in use. But now they are down to just 230, and by the end of next year, none will be on London’s streets other than the 20 or so ‘‘heritage’’ vehicles.Already, they represent only a fraction of the 8,000 buses on London roads, including those which can be operated without a conductor. Recent years have seen a resurgence in bus usage, and Transport for London — whose current commissioner is Bob Kiley, the American who previously oversaw the New York subway and the Boston Metro — thinks its modernised fleet is drawing back people who used to drive. —LAT-WP