I tried the GG-1 with a 8 car freight in tow. ZW-L red light still flashes when set to 18 volts. I then pulled out my tmcc gp9s (double a units 1 pullmor in each unit). Took the GG1 off and put the gp9s on the track.
The red light did not flash BUT the engines bucked and surged and ran horrible. I dropped the voltage down 2 volts and the gp9s ran smooth as silk. And they were not pulling any cars.