This week Derek Jewell reveals that SIMON AND GARFUNKEL came after MARTIN CARTHY , takes a took at the North American passion for songs about railroads - especially GORDON LIGHTFOOT 'S Canadian Railroad Trilogy - and, after a two-year pause, goes back to Montana again with DAVID ACKLES. Salsa, the music of New York's Puerto Ricans, gets a hearing too, along with the impressionistic sounds of EBERHART WEBER : records