Democrat Michael Dukakis, taking his presidential campaign to small-town America Friday, chose to push a theme of economic opportunity for everyone during a whistle-stop train journey. The "Duke Express" carrying the Massachusetts governor and his entourage departed Belleville in southern Illinois at mid-morning and was making stops in Bismarck, Mo.; Poplar Bluff, Mo.; and Walnut Ridge, Ark. In recent weeks, Dukakis has denounced what he calls the Reagan administration's "Swiss cheese economy," one that may be strong on the nation's borders but has "plenty of holes" in the heartland.