But....
If you're looking for the next best thing, there's the Diaz Express which launched this year (as an extension to the even smaller run the Outeniqua Power Van operator did between Hartenbos & Mossel Bay in 2013 and 2014). Each Wednesday, assuming there are a minimum 21 passengers, it runs from Mossel Bay to Great Brak and return (after a stop at the Transkaroo restaurant). Perfect for enjoying amazing view of the coastline!