I went to the December 2012 edition of Trains Magazine, which has a map showing the highest and steepest point on America's classic railroads. Here's the only grades that are steeper than 5%:
And the winner is . . . . . . . .
The Madison Incline on the Pennsylvania, which is a whopping 5.89% grade.
Second place goes to . . . . . . . . . .
Saluda Grade on the Southern Railway, which is a 5.1% grade.
Note about Saluda Grade: The official grade of Saluda Grade is 4.7% but testing in the 1970s and 1980s revealed streches of 5.1%
Also, the Cass Scenic Railroad in West Virginia has the steepest grade in use today: 11%
This excludes the Manitou and Pike's Peak Cog Railway and the Mt. Washington Cog Railway, which are cog railways.