If the old maxim that the customer is always right still has meaning, then the airlines that ply the world's busiest air route between London and Paris have a fight on their hands. The Eurostar train service linking the UK and French capitals via the Channel Tunnel is winning customers in increasing numbers. In late May, it carried its one millionth passengers, having run only a limited service between London, Paris and Brussels since November 1994, starting with two trains a day in each direction to Paris and Brussels. By 1997, the company believes that it will be carrying ten million passengers a year, and continue to grow from there. From July, Eurostar steps its service to nine trains each way between London and Paris, and five between London and Brussels. Each train carries almost 800 passengers, 210 of them in first class. The airlines estimate that they will initially lose around 15%- 20% of their London-Paris traffic to the railways once Eurostar starts a full service later this year (1995), with 15 trains a day each way. A similar service will start to Brussels. The damage will be limited, however, the airlines believe, with passenger numbers returning to previous levels within two to three years. In the short term, the damage caused by the 1 million people-levels traveling between London and Paris and Brussels on Eurostar trains means that some air services are already suffering. Some of the major carriers say that their passenger numbers are down by less than 5% and point to their rivals--Particularly Air France--as having suffered the problems. On the Brussels route, the railway company had less success, and the airlines report anything from around a 5% drop to no visible decline in traffic. The airlines' optimism on returning traffic levels is based on historical precedent. British Midland, for example, points to its experience on Heathrow Leeds Bradford service which saw passenger numbers decrease by 15% when British Rail electrified and modernized the railway line between London and Yorkshire. Two years later, travel had risen between the two destinations to the point where the airline was carrying record numbers of passengers. British airlines confide in the fact that ______.