Positive Train Control (PTC) systems supplied by PHW INC. are configured and packaged under the Advanced Civil Speed Enforcement System (ACSES) and used in several applications on the Northeast Corridor (NEC). 

ACSES enforces vehicle operation based on speed and distance (position) and on Positive Train Stops (PTSs).  ACSES uses wayside transponders installed at home signals, distant signals, and other signal block points or track locations to provide instructions on civil speed or PTSs.  An ACSES-equipped train continuously transmits a 27.115 MHz signal from an antenna mounted underneath the vehicle. 

This signal energizes the wayside transponders as the train passes over them, causing the transponders to transmit the encoded message contained in a plug attached to the transponder.  The encoded message is sent to the ACSES On-Board Computer (OBC) for processing.