In DC traction power distribution systems, rail bond damages, failures or accidental severing of individual negative feeders are generally undetected by operations as it typically leads to negligible impact on the rolling stock performance. However, a negative return circuit failure increases the overall resistance of the traction power negative return system. This increases the rail-to-earth potential and, consequentially, undesirable DC stray current activity leading to added electrolytic corrosion on affected underground metal structures. Moreover, it may lead to overload on unfaulted negative feeders. Hence, a system that continuously monitors the integrity of the negative distribution feeders is needed so that return circuit failures or inadvertent negative cable severing by construction crews can be promptly detected, located and repaired. This paper describes a negative cable monitoring system that was designed based on a substation PC distributed I/O platform and programmed using LabVIEW. The system samples the negative cable shunt signals via a remote analog input module with filtered differential inputs. The computer individually analyses each negative cable current profile in real time and a supervisory alarm is issued if a negative cable failure is detected. The system was installed at several Toronto Transit Commission streetcar traction power substations.