Agreed the "53 feet" answer is uninformed. After that, it depends on whether you are referring to American, European, or third world railroads, and whether you mean passenger cars or freight cars.
From the mid-19th Century until the 1920s, the standard American box car ("goods van") was 36 feet long. During the same interval, passenger cars were usually 55 to 65 feet long. Most European railroads retained these sizes up to the present day, while American lines tended to develop longer cars.
By the mid-20th century, the standard American box car had grown to 40 feet, and most passenger cars were 70 to 80 feet long.
In the late 20th and early 21st Centuries, box cars grew to be 60 and 80 feet long.
So you have a wide choice, depending on what effect you want to achieve. Huge amounts of information are available at your library or on the internet. Just run an internet search on "railroad car."