As well as your locos, clean your car wheels. Use the cloth/paper towel over the tracks; wet a bit of the towel with your cleaning solution and run the cars back and forth over the towel. Shift it when you get two dirty stripes on the towel; keep shifting until you don't get anymore stripes.
Alternate method is to turn the car upside down and attack the gunk with a Qtip full of cleaning solution.
For the locos, you may want to turn upside down, hold wires to the pickup wheels and with your third hand, hold a Qtip to the dirty wheels.
Kadee and a few others make little wheel cleaning brushes with brass bristles and leads that you can just apply to the loco wheels -- turns and scrapes at the same time. (You need to have pickup from both wheels on an axle for this to work; doesn't work in non-powered/non-pickup axles.)