Having recently walked and cycled this really beautiful stretch, I find it hard to imagine the loss of such an iconic walk and more importantly very bright green transport route. The number of people using this combination of old rail route and elevated timber walkways is quite substantial, particularly at weekends. There are walkers, coast to coasters, cyclists, mountain bikers, climbers and coach parties, even kayaks and canoes daring the white-water route down the River Greta: all seemingly using merely Sandwich and Kit-Kat power, not a bit of petrol or electric juice in sight. I can't think that a rail line is going to overcome the enjoyment and vitality that has already converted many to the greenest option of all, it might be perceived as a backwards step.
I would, however, like to see park and ride scheme established at locations such as Penrith and Kendal and the creation of a Renewable Energy dependant National Park Bus scheme; this would be very very welcome and establish the LDNP as one very cool green travelling tourist place to be.