Following the downward-pointed arrow of Montreal Metro signs these days means entering a new underground world of lofty marbled halls and gleaming subterranean caverns that makes tales from the Arabian Nights seem almost mediocre. Costing a king's ransom - $213,700,000 - and worth every penny of it to two million busy Montrealers, the new system boasts fast, silent transportation in rubber-wheeled trains that can swoop up and down steep gradients on smooth concrete tracks, through tunnels in the solid bedrock. Built beneath Canada's biggest city, in less than five years, this feat of engineering provides a practical transportation facility with an eye-pleasing appearance. Ready to whisk passengers along to the 1967 world's fair from 26 individually-designed stations serving 50 square miles of Montreal, work is now going ahead with extensions that may soon double its area of service. With its new sparkling subway in operation the cosmopolitan city of Montreal will be vastly eased in the problem of moving its vigorous population about their business, visitors to this enticing city will find their stay more exciting and convenient. And the city, developing rapidly into one of the showplaces of North America, will find its new Metro of great assistance in maintaining its surging growth rate as a focal point of industrial activity and transoceanic trade.