Roaring Camp & Big Trees steep grades reality or a myth?

I am confused...my math not working...
I ride the RCBTNGRR several times in the 1990s. Enjoyed the raw power uphill like in Cass WV.
Now I come to some internet publications tell me the grades are 8% between Grizzly Flats and the first switchback. Plus up to 9,5% between the switchbacks!
They run H.H.&Y.V. #3 a 37ton 1900 Heisler and Alaculsy Lbr. Co. #3 a 42ton 1912 Shay.
According to the manufacturers catalogs these lokies can haul on perfect straight track about 55-60 tons 8% and 35-40 tons 10% uphill.
The trains use usually 6 cars. Lets say one car is only 10 tons empty plus 100 passengers for the train (10 tons) we are at a minimum weight of 70 tons! Probably more because I think the cars are heavier.
Even with this very light estimates this is impossible on 8% or steeper grades.
I think in 1963 they made a rule of thumb calculation for the public and within the decades some publisher adds the one or other percent to the grades because it sound good.
As an european living in asia right now it is not possible for me to go to Felton soon.
Maybe someone here can check. There are a lot of hiking and bicycle apps for our smartphones including track and height profile for the way. Maybe someone can use this on the next trip to Bear Mountain and post it here.
My estimate is 4-5% maybe 6% between the switchbacks.
Thank you.
PS Also the Butte & Plumas #4 (West Side Lumber #7) 60-3 Shay could only pull about 60 tons on 10% grade.
PS2 Is it possible the shop boys was at the friendly locomotive tuner around the corner? Installing high pressure boiler (300psi), lower gearing (4:1) and smaller wheels (20")... hahaha