I have about four feet of easement into the grade and at the top of the grade. Since the longest "locomotive isn't more than 24", that gave me plenty of room to ease into my 2.5% grade and it all seems to have worked out. I've run most of my bigger locomotives on the layout, and no issues on the grades.
With steam, you don't have to consider the tender in the length for easements, it's separately vertically articulated. 
