To access Penrith station, trains need to use the Network
                           Rail (West Coast Main Line) tracks crossing the M6 by a long bridge near Junction 40 of the M6. Heritage or light rail trains would probably not be permitted to do this for many technical and safety reasons. 
As an independent Railway, Keswick to Penrith would need its
                           own trains, crews, maintenance depot, ticketing and revenue systems. These all
                           add enormous costs without increasing revenue.
Being
                           forced to change trains at Penrith would also make long journeys less attractive. 
Herittage railways rely on volunteers - none have been able to find enough volunteers to
                           run a reliable every day public transport service.
THe
                           CKP proposal to build and manage the track and stations, with established Train Operators bringing in trains from other areas
                           each day is much simpler, cheaper and more effective.
The
                           train service in the CKP proposal would be part of the national network with through tickets, "visible" to potential
                           travellers all across the UK and coming from abroad. 
A "light" railway would not be able to carry freight or heavy main line passenger trains - Keswick would
                           be permanently isolated.