When it was in active service, Joliet Union Station was at the junction of the former Rock Island Line, Atchison, Topeka, and Santa Fe, and Alton Railroad main lines. Therefore, it has two different milepost numbers depending on how far each route is to Chicago. On The Metra Rock Island District Line, Joliet is away from LaSalle Street Station, the northern terminus of the line in Downtown Chicago. On The Metra Heritage Corridor and Amtrak routes, away from Chicago Union Station, the northern terminus of the line.