I am not sure which term is more common when people talk and refer to metro station. Somebody told me you should use subway instead of metro and people use subway in everyday talking. Is this true? I am living in Washington,DC.
Each region has a name for their transportation system, so it varies. For New York, I have mostly seen "subway", for DC it's "metro", for Boston and Pittsburgh it's "the T", for Chicago it's "the L".
In terms of strict dictionary definitions, both terms imply underground trains. Subway is a generic term, but metro is more localized to certain cities.