Metro Scene never has been fond of negative advertising -- the kind that tells readers or listeners how awful the competition is. It took great restraint to avoid attacking in this column some print and broadcast ads run recently by a commuter airline.

The airline flies the Washington-Philadelphia run, and instead of listing its pluses, it chiefly criticized Amtrak, saying those time-consuming trains are cramped and uncomfortable -- although the trains really are nice to ride and fast.

As one who has traveled on both airline and train between Washington and Philadelphia, and as a seasoned global traveler, I found: (1) the trip by ground from Philadelphia International Airport to the center city to be one of the most unpleasant and time-consuming by bus or cab anywhere; and (2) the arrival or departure at the 30th Street train station to be among the most convenient ever available near a center city.

In a word, the airline ad was unfair, if only because airport-to-airport time comparisons fade, on short-haul trips, between figures comparing travel from one center city to another. The Washington-New York route, where the train often has an advantage, is a good example.

Now, however, we have word that travelers arriving at Philadelphia International Airport can ride a new high-speed rail line, chiefly using Amtrak rails, directly from the airport to center city. Trains started running Monday from the airport every half hour, from 6 a.m. to midnight, from four domestic airline terminals to the 30th Street station and three other stops.

The new service, while it chiefly uses Amtrak rails, is operated by Philadelphia's counterpart of our Metro system -- and to a degree, it puts Amtrak in competition with itself. Only four U.S. airports have direct-rail, rapid-transit service to their cities' downtowns -- Washington National, Cleveland Hopkins, Chicago's O'Hare and now Philadelphia International. Boston and Oakland have shuttle buses to nearby rail transit stations.