Kevin,
The 12 was actually built for road operations.
HOWEVER, the 12 was built for road operations on the Island of Maui to haul cane from the cane fields to the processing factories for the sugar companies. There were no 4% grades on the Kahilui Railroad. The gauge may be the same, but the application was much different.
Remember that Steam Locomotives were not an "off the shelf" item like diesels are today. Each one was designed with a specific purpose in mind.
Why do you think that the 9 was so big for its era?
Rick