The historic linkup of a tunnel under the English Channel is only one step in a larger plan to connect the great cities of Europe with a network of superfast trains.

Rail planners envision sleek trains rocketing through the French Alps, across German plains and, of course, under the channel.On board, they hope, will be travelers fed up with Europe's congested airports and highways. By 2010, they could be riding from Madrid to Frankfurt in less than 10 hours, compared to nearly 24 hours now.

"Train will soon allow what only plane made possible," predicts Karel van Miert, the European Community's transport commissioner.

He recently introduced an EC plan to create a $205 billion Western European high-speed network over 20 years. The 15,500 miles of electrified track would carry trains running at 120 to 200 mph.

The channel tunnel, which linked up in early December, will join London and Paris with a 31/2-hour train ride in 1993. It will enable a London banker, for example, to hop a train to Brussels, Frankfurt, Madrid, Milan or dozens of other cities.

Environmental worries, ethnic squabbles and high construction costs could derail parts of the proj-ect. Financing remains uncertain and the plan still needs approval by the European Commission, the community's executive body.

"When it will really be done, I don't know," said Hughes Corbeau, a rail planner for the French Transport Ministry. "It's going to cost a lot, and demands concerted efforts from public powers."

High-speed rail systems - minimum speed 120 mph - already built or under construction in Britain, Germany, Italy, Spain and France form the core of the proposed network.

Also foreseen are new high-speed systems or linkups by rail or ferry in Belgium, the Netherlands, Greece, Portugal, Switzerland, Luxembourg, Austria, Ireland and Yugoslavia.

The technological star and pacesetter for high-speed rail is the chisel-nosed French TGV, or "train a grande vitesse."

TGVs went into operation between Paris and Lyon in 1981, racing along at 174 mph. They halved the rail time between France's two leading economic centers to two hours, and boosted ridership between Paris and southeast France from 12.2 million to 22.1 million in nine years.

France expanded the TGV network last year with a second generation of more powerful trains running east-west. They cruise at 186 mph, but one engine set a world rail speed record of 318 mph.

The TGV runs at high speed only on special track. On the 31/2-hour trip from Paris to Geneva, it slows to 125 mph on conventional track after Lyon.

Railway directors hope high-speed trains can win back airline travelers on trips of 600 miles or less. Hours-long airport delays are increasing, especially at London, Paris and Frankfurt.

Transport studies show European air traffic increased 56 percent from 1975 to 1986 and could double by 2000.

The complete train network will not be easy to accomplish, if the planned hookup of London, Paris, Brussels, Amsterdam and Cologne serves as any example.

It already has intergovernmental approval and the route lies over flat terrain, so it should be among the easiest to build. However:

- Britain refuses to lay special TGV track in the lush, highly populated countryside between London and the channel tunnel, adding 30 minutes to the trip.

- Regions of Flemish and French speakers in bilingual Belgium are squabbling over who will get the special track and thus reap the most economic benefit.

- In the Netherlands, the shortage of dry land makes it impossible to lay the long straightaways TGVs need to reach full speed.