The Los Angeles Metropolitan Transportation Authority (LACMTA), like many other transit agencies throughout the country, is currently addressing the stray current corrosion problems on its rail system. Numerous capital projects have been released by the authority for the rehabilitation of their corroded infrastructures along their right-of-way. In addition, new maintenance procedures have been implemented to minimize corrosion problems to the rail and utility infrastructures located adjacent to the electrified railroad.
The corrosion effect on rail infrastructures is often overlooked by most electrified railroad authorities because an immediate corrective response is not necessary for train movement during rail operations. The corrosion process is a natural process that occurs slowly and continuously throughout the life span of all materials, and may be accelerated due to stray currents from the railroad electrification system.
Several key locations along the right-of-way have greater impact from stray current corrosion issues: at street crossing due to poor rail insulated boots, switch machines and accumulation of brake dust near passenger platforms. Other significant locations that suffer from stray current effects are overhead and under grade bridges, tunnel structures, rail spikes, fire suppression pipes, sewage pipes and underground feeder cable connections. Moreover, stray current can also cause other vital systems such as signaling systems and communication systems to malfunction.
The other aspect of stray current is intentional discharging of stray current into earth ground through Negative Grounding Device (NGD) to maintain negative rail over voltage level for safe operation and reliability of rail service to the public. Typical negative rail voltage with respect to earth ground in operation is over 100VDC which is over the recommended voltage of 50VDC.
Corrosion problems can be controlled by the implementation of a cathodic protection system, proper inspection of the running rail, impedance bond connections and proper maintenance of the cathodic protection system and negative grounding devices.
The purpose of this paper is to address corrosion issues associated with DC electrified railways, recommend maintenance practices to control stray current sources, recommended maintenance practices for cathodic protection systems and discuss the balancing act to control negative rail over voltage versus intentional discharge of stray current to earth ground.