THE AGE when we built great city parks, or parkways, or boulevards is over and has been for forty years or more. Anyone raised in New York City or Boston knows how much these cities were embellished by the park and parkway designers and builders, and pre-eminently Frederick Law Olmsted. Our efforts are devoted these days to retaining as much as we can of their achievements, rather than adding to them. But occasionally a new opportunity to add to our parks arises. The last major piece of “undeveloped” land on the island of Manhattan, the former railroad yards on the Hudson River between 59th and 72nd Streets, will, on the basis of plans worked out between developer Donald Trump and community groups, provide opportunity for a park on the Hudson River. What will it be? What can it be? This was the challenge placed before student landscape architects in five major schools, and the subject of an exhibit of their answers in New York City last March.