What’s Old is New Again: MARTA’s Improvement to Track Circuit Reliability and Maintenance for Mainline Interlockings

The Metropolitan Atlanta Rapid Transit Authority (MARTA) has replaced double rail audio frequency (AF) track circuits on its Northeast and South Lines with single rail AC track circuits in an effort to improve reliability and maintenance of this critical subsystem for the overall train control system. A total of 14 separate interlockings were converted to AC track circuits and long wire cab signal loops which are being used for all speed command transmission to transit vehicles in the interlockings. The use of AF track circuits in interlocking applications has created problems with traction negative return current imbalance, premature impedance bond failure, EMI issues, non-continuous speed command transmission, reliability, and maintenance. While the signal suppliers have developed numerous types of track circuits to meet the needs of the industry, MARTA has decided to use older technology resolving all the problems and issues with AF track circuits. The new AC track circuits in MARTA mainline interlockings have been extremely successful and it is MARTA’s desire to convert all mainline interlockings from AF track circuits to AC track circuits with long wire cab signal loops. As a result of this, two additional contracts are in progress to meet this requirement. This technical paper will benefit other transit agencies that are considering signal system upgrades by discussing the advantages and applications of using single rail AC track circuits and long wire cab signal loops on MARTA as outlined above. The basic history of AC track circuits is discussed as well.