The March 2019 Scotland Freight Joint Board's report "Industry Growth Plan for Rail Freight" contains detailed analysis of the present limitations of the whole Scottish rail system in terms of freight.
On the Far North Line the report sees the potential for one more freight path each way and commodities such as timber (incl biomass), intermodal, metals and fuel (LPG). However, it mentions cost implications for timber traffic if double handling is required i.e. if there is no direct rail access at the source or destination.
The factors affecting freight are the length of passing loops, the per-axle weight restrictions on structures such as the Oykel Bridge, width limits for containers and speed restrictions for Heavy Axle Weight (HAW) vehicles. The last item is of particular importance to allow freight paths to fit in with passenger services and the aspiration is to remove these restrictions.