Most Transmission corridors are easements across private lands. The easement language is specific to the electric utility use. Neither the owner of the land or the easement normally grants permission for any other use. Same principals apply to Transmission corridors on public lands as well, except that you might have a right to access the public lands under the transmission corridor for another legitimate purpose.
On Federal Lands, the utilities have to do environmental clearance on all access routes to and along the line. It is amazing how many endangered bugs, birds, bunnies, and flowers suddenly move into these corridors. Driving along or doing routine maintenance on the line then really inconveniences all of evolutionary challenged things. Mud bogging under the transmission corridor is not very PC.