Tunnel Hill just north of Owaka has some of the last remains of the old river railway through the Catlins. Yes it ran all the way from Balclutha to north of Papatowai (Tahakopa) till 1971 ferrying timber.
The tunnel is the most southerly one in the World. Built using handmade local bricks it was completed way back in 1895.
It is just a short 10 minute walk and make sure you take a torch (you might even see glow worms).
If you want to follow the tracks then the only real bit is the heritage trail next to Matai Falls.