The Center of Saint Petersburg is the area of main interest to the traveler, home to most attractions of the metropolis. It's bound by the Neva river and the Obvodny Canal, on a set of islands formed by numerous rivers and channels.
The Moscow and Vitebsk railway stations are in the center; if you arrive by train, you can begin your sightseeing directly from the station. Baltic Railway Station is located just outside the center, but it makes sense to take the metro : Baltiiskaya or shuttle bus to the more interesting places (e.g. Palace Square). Finland Station is within walking distance from the Liteiny Bridge, to other parts of the center it's better to get into : Ploschad Lenina (Площадь Ленина) metro station. From Ladozhsky station it's better to take the subway :Ladozhskaya (Ладожская), because it is farther from the center, and the frequent traffic jams on Zanevsky Avenue.