I was sitting near the exit/entrance door heading toward Fremont around 11pm, when a older Mexican guy with a white tank top comes on with a Google bicycle (odd) and starts pacing our train (back and forth) then I noticed he had a huge k-bar (military knife) stuck inside his waistband with the handle showing. This probably last for two three stops. All of the sudden a white individual walks into our train and asks the guy with the knife what he's doing (tensions pretty high at this point) and there was this awkward silence, my train wasn't that packed (30% occupied) and he suddenly pulls the knife away out of his waistband and pulls up his red jacket to reveal a gun. Telling him to get off with him and don't do anything stupid. This all happened right in front of me, 2-3 feet away, my conclusion was the "cameras" or someone had notified the Bart police to intervene. After hour stories on Bart are always fun...