A 20% grade is probably unachievably steep for ordinary railways. Even a 5% grade is considered remarkable.

Wikipedia says (https://en.wikipedia.org/wiki/Saluda_Grade):

Saluda Grade is the steepest standard-gauge mainline railway grade in the United States.[1] Owned by the Norfolk Southern Railway as part of its W Line, Saluda Grade in Polk County, North Carolina, gains 606 feet (185 m) in elevation in less than three miles between Melrose and Saluda. Average grade is 4.24 percent for 2.6 miles (4.2 km) and maximum is 4.9% for about 300 feet (91 m).

Unless you're gonna build that mountain style with gear drive and toothed tracks, you're probably looking at 5% grade and a 20km ride, which drops you down to ~140kW for 2 hours.

Ion the other hand, it seems you can get 11 thousand tonnes in a coal train:

https://www.australianmining.com.au/news/%E2%80%8Bnew-coal-t...

So on my 5% grade track I could get 1.5MW and if you can get your 20% grade to work that'd be just over 6MW