Without seeing pictures, I believe the "brass ground contactors" that you are referring to are known as axle wipers and provide a ground connection through the wheels and outside rails for the lights to work properly.
First, check to make sure that neither of the wipers is broken or not in contact with the axle.
Second, check to make sure that there are no broken wires or wire connections anywhere in the vicinity of the wipers or at the bulbs. Because they rotate with the turning of the trucks, the wires get a lot of wear and stretching and the connections are easily broken.
Third, the wipers tend to get carbon, grease and dirt build-up where they contact the axles. Use some electrical contact cleaner to remove any discoloration and clean the axle and wiper thoroughly.
Finally, the wipers are easily bent and might not make contact with the axles. If so, they can be carefully bent back to make contact.
If all else fails and you are not satisfied with the lighting, you can always install LED lighting.
