The Secret Empire

The Secret Empire was a sci fi western serial in which a US marshal discovers a futuristic alien city hidden beneath a town in the old west.
It was shown with two other chapters each from a different serial, an adventure series called Stop Susan Williams, and a vampire thriller called The Curse of Dracula.
The three chapters would always end on a cliffhanger to be resolved in the next episode, and so the hour long show combining the segments was transmitted as Cliff Hangers!