Police are investigating a taxi-jacking.


  Detectives say around 1AM Monday, a cabbie picked up a fare downtown who asked to be taken to the northeast side.


  When the cab reached the 1604 and Judson area, the passenger pulled a knife and put it to the driver's throat.


  The cabbie jumped out of the taxi to get away, and the knife-wielding suspect then jumped into the driver's seat and took off.


  The cab was found abandoned about a mile away.  The cabbie will be okay...the suspect remains at large.