Please note that the atmospheric railway can be made to work with modern materials, Brunel ahead of his time as usual, though I don't think it was actually his invention, he just adopted it. The main problem was rats eating the greased leather sealing flaps. It was in service for a year or two.
The reason that much of the railway west of Exeter had such steep gradients was that it was expected to use the atmospheric system, which could cope with them better than steam haulage. In 1911 it was a close vote as to whether the Great Western would turn over to electric propulsion for this part of the network.