I use the CRC "Safe for Plastics" contact cleaner on a micro fiber rag. This stuff evaporates pretty quick so I can get about three feet before I have to spray the rag again. This makes for a little my work but the results are well worth the effort.
I recently have laid enough track on the upper level of my layout that I can "run trains" to test. I first just tried to run a couple of locos on DC just for testing. They did not run well at all due to dirty track. After wiping with the CRC cleaner it was like a night and day difference. Trains still run fine now from the cleaning done 6 weeks ago.
Polished rails are much better than scratched rails, so I never use any abrasive type of track cleaner. Using any abrasive on track leaves micro scratches in the rail which are a great place for that "black gunk" to live.
I have a friend who cleans his track with a "Bright Boy" cleaner. He is always griping about how he has to clean his track every time he wants to operate his layout............ go figure. His track cleanings last about 2 to 3 days before needing another cleaning. I have tried to get him to try the CRC cleaner, his answer is "My trains run fine after I do the Bright Boy cleaning". Wheel gunk build-up is also a byproduct of abrasive cleaning.
Track cleaning cars with the block dragging on the rails are fine if the block is covered with with denim, corduroy or other similar fabric that will not snag on points. Just a plane masonite block will work if not weighted to incur scratching on the rails.
Latest recommended cleaning solutions from MRH magazine:
Items with a "strike" line through them are NOT recommended due to other problems associated with them.
Note the WD-40 "CONTACT CLEANER" is much better than the usual WD-40 water displacer/lubricant.
Items with a lower "Dielectric Constant" are best.
Hope this helps....John