I question if it's the pickups that are the issue. I wonder about the conductivity of the wheels to the frame, you have four traction tires, so only the remaining four wheels for track connection. When I measure conductivity between the wheels on the two trucks, I get varying readings, I'm thinking that they're more likely the issue than the center rail pickups.
In any case, there's no reasonable possibility to add pickups that I can see, where would you put them? I'm wondering if axle wipers would help...
