Eleven times a day starting at 6 a.m., a 13-car diesel train rolls out of Kowloon Station here carrying passengers 21 miles to the border of once-forbidden territory: China.

The early morning trains generally haul a mixed load of "overseas" and Hong Kong Chinese, Western business people, "invited guests" of various nationalisties and lately even an occasional group of American tourists.

But although the State Department's travel ban has been lifted since 1971, and China has at last been "opening up," there's one kind of American traveler for whom getting space is as unlikely as getting wings. That's the would-be sightseer who comes to Hong Kong thinking all he has to do to visit China is picking up a visa and hop aboard.

Considering the proximity of the two cities, catching the Hong Kong-Canton train to the border and into China would seem easiest, most logical way to visit the People's Republic. But in fact, a spur-of-the moment visit is practically impossible.

Smiling but brusque officials at the China International Travel Service, the government agency that controls all tourism, reportedly listen of fumbling requests for entry information almost every day. However, so far as anyone knows, without prior arrangements their final response is invariable the same to other nonresident foreigners as well as to Americans: No visas are available.

Persistence leads to diplomatic evasion: "How long will you be in Hong Kong?" Even unstated, the implication is clear: There is no way you can be around long enough.

So how can Americans visit China? What's the most likely way to get permission?

First, one must understand that the Chinese approach is not your typical bring-on-the-tourists approach. An airline executive who's been "inside" four times says tourism has a low priority in the People's Republic. It's not a money-making propositions, and there are insufficient facilities and motivation to allow it to become one. What space is available, then, is allocated primarily in ways that allow the Chinese to send a message to the world - through "friendship societies," study groups and trade delegations.

Still, more and more "pure" tourists (those who aren't part of a political or semipolitical delegation) are being admitted. Knowledgeable observers say, however, that the number is fixed by an unpublished quota. The yearly total, they say, depends on what's available in terms of hotel space, buses, interpreters and guides. Places then are distributed on a Friendship First basis. In 1975, for instance, Brazil, Mexico and Argnetina reportedly were allowed 700 tourist visas apiece while Switzerland, Italy and Austria got only 200 each. No one seems to know the U.S. quato (nor will China Travel Service admit there is one), but chances are that it, too, is small.

Tourist visas frequently are further controlled by tour operators or travel agencies which, one way or another, have established a "special" relationship with China Travel Service.

Under these circumstances, it would appear that the traveler's best chance of visiting China is to sign up for a package tour. The fact that Peking won't tell who's likely to have a tour makes the game more challenging or more annoying, depending on your frame of mind.

Most recently eight cruise ships - the Rasa Sayang, Royal Viking Sky and Sea, Rotterdam, Queen Elizabeth II, Canberra, Danae and Marco Polo - were granted permission for passengers on certain trips to enter China.

Exceptions have been made allowing Americans to join tour groups originating in another country. Generally, though adding a nonnational is thought to jeopardize acceptance of the group. As a result, it is not noticeably encouraged and Air France, for one, will not even consider foreigners' applications at present.

Are there other possiblities? Yes, more so than at any other recent period, although that's still not saying much. If, for instance, the world-be tourists are the rare persons with foreign friends residing in China (for all practical purposes, diplomats or journalists), the friends can ask to receive them as guests. Success is rare, though; one correspondent is still waiting for an answer six months after asking.

There are iffy indications that a tourist may occasionally be able to get in by being a "transit passenger" travelling by air via Peking. (One airline spokesman's opinion is that a tourist's chances in general improve if he or sheis flying via CAAC, the Chinese national airline.) The transit passenger may be able to get a transit visa valid for up to five days.

To apply for a transit or a full tourist visa on an individual basis, the procedure is largely the same. According to the latest "Travel Gazette" (published irregularly by China International Travel Service and distributed only in Hong Kong), foreigners themselves or their travel agents can apply either to the Head Office, China International Travel Service (LUXINGSHE), No. 6 Dong Chang' an Avenue, Peking, or through the Chinese embassy or liaison office in their home country. All signs point to the Peking route as best, however. A spokesman in the Chinese Liaison Office in Washington, (2300 Connecticut Ave. NW. 797-8878) says permission to visit China should be sought from Peking before applying to the Liaison Office for a visa.

So get set for a wait. Even a pre-"approved" European tour scheduled for May began "final" paperwork in January. Some groups and individuals never hear anything at all. And one "approved" Swissair group was canceled the day before its scheduled departure.

You should nonetheless supply full details at the outset: full name, nationality, age, sex, occupation, number of companions, choice of itineries and cities,days of stay, exact date and place of entry-exit, means of transportation and list of languages spoken. Among other things, you need to be able to make an educated guess on what places are open to tourists, just to avoid an automatic turndown. You further need to ignore any bourgeois concern such as how much the whole business is likely to cost. Unless you're applying in connection with an announced tour, about all you can learn for certain in advance are air fares.

But keen-eyed China-watchers report a few guidelines:

It's generally easier to obtain an off-season visa, November-March, and very difficult to get one for July-August. In the two summer months, the Chinese hotels are normally full of student groups.

Approval is most easily given for arrangements limited to the Big Three cities: Peking, Shanghia, Canton.

Preffered duration for a visit is two weeks.

Because groups take less servicing once inside the country, their applications are favored over applications from individuals, and groups of 24 appear to be looked upon the most favorably of all.

Group requests are best made on the letterhead of a school, club or association.

Most important if the phrasing of the request. Never, it seems, should one make the error that a recent business traveler did when his letter of credit did not call the country by its full name, the People's Republic of China. ("We have reason to believe this may not have been a genuine mistake," he was told sternly.)

Nor does it hurts if one lets his or her "correct thinking" shine through. A certain floridness denoting enthusiasm for social progress, agriculture, communes, revolutionary progress and , above all, devotion to international friendship and understanding will not be taken amiss.

Especially eager applicants would probably find it useful to bone up on their subjects by ordering reading materials from the People's Republic publishing house, Guozi Shuian, P.O. Box 399, Peking, or an agent such as Peace Book Co., Chung Shang Building 73/8F, 9-10 Queen Victoria Street, Hong Kong.

If, after submitting the perfect reasons for wanting to visit China, you are chosen, what then? What's waiting in the curious country of Maoism, the once many-splendored setting for a civilization that reached stories heights in arts, scholarhip and culture?

An exasperating, irritating, fascinating, eye-popping spectacle - in short, a grabber. Which may be one reason visa applications keep on coming.

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