Mystery Mountain

Mysterious attacks by an unknown criminal who calls himself The Rattler have stopped the railroad's attempts to tunnel through Iron Mountain.
Railroad detective Ken Williams (Ken Maynard) investigate with help from the railroad's arch rival Jane Corwin. Jane, the heiress of The Corwin Transportation Company, may be a secret agent of The Rattler or an even more diabolical threat playing both sides against the middle.
The Rattler's poison darts target anyone who comes too close to his futuristic cave hideaway. (official distributor synopsis)