[6] Train tracks run down the middle of 2nd Street in St. Marys, and freight trains running through the middle of downtown St. Marys are a common sight.

It is one of the few remaining towns in the United States where freight trains actually share city streets with automotive traffic.

St. Marys is located along the Ohio River at the mouth of Middle Island Creek.

[8] According to the United States Census Bureau, the city has a total area of 1.02 square miles (2.64 km2), all land.

There were 954 housing units at an average density of 935.3 per square mile (361.1/km2).

31.2% of all households were made up of individuals, and 16% had someone living alone who was 65 years of age or older.

30.9% of all households were made up of individuals, and 18.8% had someone living alone who was 65 years of age or older.