East Berlin was officially called Berlin , Hauptstadt der DDR ( `` Berlin , Capital of the GDR '' ) , or simply `` Berlin , '' by East Germany . This circular line connected all train routes heading for West Berlin and accommodated all domestic GDR traffic , thus directing railway traffic into East Berlin while by-passing West Berlin . Commuters in the East German suburbs around West Berlin now boarded Sputnik express trains , which took them into East Berlin without crossing any western sectors .