Mass Transportation Administration chief Richard S. Page said yesterday that the Carter administration is prepared to support new heavy rail (subway) transit systems if they are carefully planned and cost effective.

Page, at a meeting at the convention of the American Public Transit Association here, also said that the administration is preparing legislation that would recognize the hand-in-glove relationship highways and mass transist systems have in urban areas.

ALthough Page insisted his statement on rail systems was nothing new, it was the clearest pronouncement on the subject since President Carter's now-famous handwritten note to Transportation Secretary Brock Adams that many new subways were "grossly over designed."

"I agree with that statement." Page said, "and I think every taxpayer does. BART (San Francisco's Bay Area Rapid Transit) and Washington's Metro have created substantial questions about heavy rail. But there are high density corridors in which heavy rail may be the best" transportation solution," he said.

Page also dismissed a recent report from the Congressional Budget Office calling, subways less energy efficient than buses as "a bum rap" and "misleading" and suggested that it was statistical games playing.

Rail transist, Page said could expect favorable considerations:

If proposals are part of a comprehensive locally supported, transportation plan for whole system - highway, bus and rail.

If alternative means of transportation had been studied for every corridor and rail chosen as the most cost-effective.

If the system could be built incrementally with such segment capable of independent operation. That is a direct outgrowth of the Urban Mass Transportation Administration's problems with both BART and Metro, which were designed as giant 1- and 100-mile systems that do not operate well at shorter distances.

If there is a realistic estimate of operating costs and a sound financial base to pay them.

If construction contracts are awarded with fixed ceilings.

If UMTA is permitted an as yet undefined role in monitoring design and construction.

Page pointed out that in recent weeks Transportation Departmental has released more than $400 million for subway system projects in New York, Boston and Chicago.