Freddie Laker is currently fighting two battles: one to begin his innovative, low fare air service between New York and London with as few restrictions as possible, the other to preserve the trademark SKYTRAIN.

The first battle, already into its seventh year and nearing its end, may be easier than the second.

SKYTRAIN has been registered with the U.S. Patent Office as a service mark to describe the kind of air passenger service Laker plans to offer. But Laker, the chairman of Laker Airways Ltd., and his lawyers are worried that the word has become so popular that they might lose it.

"If SKYTRAIN falls into the vocabulary as a synonym for a low fare, non-reservation service, we will lose our unique property right to it," Robert M. Beckman, Laker's Washington attorney said. "Then everyone could use it.

"A lot of money is spent in trying to create in the public mind a mark . . . we think SKYTRAIN is valuable and we are doing everything we can to protect it, so far successfully," he said.

Protecting it means remaining ever vigilant to its misuse and acting to stop it. For instance, several other airlines have tried to use the word, including World Airways, Overseas National Airways and Braniff International. When Braniff asked the Civil Aeronautics Board recently for authority to fly between Dallas/Ft. Worth and London, it said it would offer a "Texas Skytrain" fare. After that incident, Beckman said Laker attorneys informed each airline that the word was a trademark and asked them to refrain from using it again.

Others who have unwittingly used it incorrectly, including the Civil Aeronautics Board, the Justice Department and newspapers, have also received letters from Laker attorneys pointing out the proper usage of the word. It is supposed to be different in appearance from any words or letters that precede or follow it. In normal text that requires being spelled in all capital letters. It is to be used only as an adjective for the generic "air passenger service," or "non-reservation air passenger service," such as SKYTRAIN air passenger service.

Many companies have lost their rights to certain trademarks by failing to keep words they "coined" to described a new product or service from falling into common use. Such words as escalator, evelator, linoleum and cola were once trademarks of companies, Beckman pointed out.

The Coca-Cola Co. has been struggling for years to keep the words Coca-Cola and coke as their own, just as Xerox Corp. has fought for years to keep people from saying they want to "xerox" something. Eastman Kodak Co. is waging a similar fight to prevent people from using Kodak as a synonym for camera.

It appears that Hoover Co. has lost its property right to its name in England, Beckman said. The British say they "Hoover" a rug, rather than "vacuum."

"It's very difficult to preserve a trademark," he said. "I'm constantly reviewing every piece of Laker advertising to make sure he doesn't misuse it."