As you pass over the traffic on Dallas' clogged freeways, lower your paper for a second and just look at them down there, sitting in their cars, spasmodically inching along as you zoom past overhead. Then thank us for telling you to take the Trinity Railway Express from Dallas to Fort Worth's gleaming T&P station. The ever-growing popularity of this line may be linked to the blossoming appeal of downtown living; downtown Fort Worth, that is. It also may be that the trains are invariably on time to the minute, 65 of them from Union Station to the heart of Fort Worth's vibrant central district. And at $4.50 a ticket, not only is it faster than driving, it's also cheaper.