The Abandoned Mines run under the town of Stonecliffe. The entrance to Tyaardos can be found here; located near the entrance to the Abandoned Mine (turn left when the rails end and walk through the wall).
Abandoned Mine Caverns
The Abandoned Mine Caverns are inhabited by Level 7 Goblins. It is recommended that you be at least Level 5 before entering.
It can be found by simply walking further into the Abandoned Mine.