Hola Fish,
Madrid's Atocha train station is closest to the City Center, Prado & Retiro Park. Chamartin is in the northern part of Madrid, near the Plaza de Castilla, off the Paseo de La Castellana. You use Chamartin to go to sites North of Madrid like Santiago de Compostela, Leon, Bilbao, etc.
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The world is a book, and those who do not travel read only a page.
--St. Augustine (354-430)