Puling power will remain constant, while what affects the amount of work it can do is the effective grade.
Tractive effort remains constant. It's the drag that increases. So it's not really the pulling ability which is affected, although that is the apparent affect. What's actually happening is that the the same number and weight of cars requirement more of that tractive effort to move it.
Connecticut Valley Railroad -- A Branch of the New York, New Haven, And Hartford
"We don't stop playing because we grow old. We grow old because we stop playing." --George Bernard Shaw