IMAGINE a passenger train that is spotless, runs on schedule, with curtains and seat covers of white lace, chairs that swivel to take advantage of the view, tables with tea mugs separating each seat, tea refills brought to your seat without your asking, sleeping compartments which are spacious and comfortable and dining cars that are efficient and serve freshly cooked meals.

No, they are not Amtrak trains. They are the trains of the People's Republic of China, reputed to be among the finest in the world.

Better yet, imagine that the Chinese railroads, built primarily by the British early in this century, were paid for by Chinese government-guaranteed bonds sold abroad; that these bonds, still held by many individuals, including Americans, have never been repaid; and that we are nonetheless talking today about extending large amounts of credit to this same nation.

The value of these Chinese rail bonds, plus lesser amounts of straight Chinese government bonds, is about $220 million, obviously a minuscule sum in relation to China's resources.

China's proposed "Four Modernizations" (of agriculture, industry, defense and science and technology) will cost an estimated $800 billion by 1985. So the face amount of its unpaid foreign debt is about one-fortieth of 1 percent of that anticipated outlay.

Old Chinese bonds may seem like a modest issue in the context of potentially momentous changes in U.S. China relations. But Treasury Secretary Michael Blumenthal's coming trip to Peking, intended to settle questions about frozen assets still held in both countries and to set a framework for future trade relations, should suggest that such matters are not trivial.

If the old Chinese bonds remain ignored, what security can there be for future loans from the West? What would happen to such funds if the current Teng regime slips from power, not exactly an impossible occurrence? Would those debts be honored?

Under international law, successor regimes are responsible for the debts of their predecessors, and communist regimes are no exception. In fact, one by one, other communist nations have been recognizing and settling their outstanding bonds, if not always on the most generous terms.

In part, this is because such settlements soon pay for themselves by lowering the interest rates on later borrowings. For nations as well as individuals, repayment of debts is universally regarded as one of the best indications of credit-worthiness.

It is really in China's own interest, then, to finally settle the foreign debts it inherited when the communists took over the mainland in 1949. It is certainly in our own interest to press for such settlements as a sign of the good faith and confidence required in future trade relations. And that is to say nothing of the simple justice due those who all these years have been holding the bonds that paid for that remarkable Chinese railway system.