About a 15 minute walk from the train station you will reach the Belvedere. The place is gorgeous! I would return over and over just to walk the grounds.
There are manicured gardens between the lower and upper palaces. The combination of the palaces with the fountain, the iron gate and the sculptures are definitely a good reason to add this place to your visit when in Vienna.
It was Prince Eugene of Savoy’s old residential complex. The lower Belvedere was his residence/castle while the upper Belvedere was used for presentations/receptions.
Today, both buildings present Austrian art.