BELGRADE, Sept 22 (Reuters) - Serbia has signed the extradition papers of a key suspect in the Madrid train bombings and will be transferring him to Spain shortly, a government official said on Thursday. Abdelmajid Bouchar, a 22-year-old Moroccan, was detained on June 23 on a train travelling to Belgrade and held for a breach of immigration law. Serbian officials confirmed his identity in mid-August after checks with Interpol and Spanish police. The official, speaking on condition of anonymity, said the extradition date would not be made public for security reasons. "A statement will not be issued before he arrives...