The Mt Rennie Tunnel runs under Narrow Neck Plateau, connecting the Megalong Valley with the Jamison Valley.
It was built so oil shale from the Glen Shale Mine, on the north side of Narrow Neck, could be transported to the Blue Mountains standard gauge railway using the existing cable haulage facility, the Katoomba Incline on the south side, in the Jamison Valley.
The tunnel is about 400m long, 3m wide and about 1.2m high for most of its length and is just large enough for the double track tramway. Thick orange mud makes the traverse of the tunnel daunting.