If the transition is too sharp the pilot (cow catcher) on a die cast steam locomotive can short out on the center rail/
Also, it is the combination of rise and length of a grade and not just the grade. You can climb a much steeper grade if it's short enough as Rich has demonstrated. Only a portion of the total train is on the grade at any time.
The theoretical limiting grade for a locomotive with all wheels powered for steel wheels on steel rails in 25%.
Jan