With such a small electrical contact patch for engines, clean track is essential.

Cleaning with abrasives: abrasives can definitely "clean" the track. They can cut through corrosion, rust, etc. There are negatives though, use sandpaper and it will wear down the rails, and can put lengthwise "grooves" in the rail that will help hold dirt, etc. The "eraser" type of cleaner can leave bits of gummy stuff on the track, and leave the rails greasy.

Cleaning with solvents: This method will not really remove corrosion or rust, but will remove oily grime and dirt. Be careful that the solvent does not eat up the plastic! I would suggest alcohol but be careful, "rubbing" alcohol usually contains glycerin, which can leave a gummy residue.

I will be experimenting with various types. I have heard the Aztec cars work well.

"Electronic cleaning": this is not really cleaning the track, but a method of using a temporary high voltage to conduct "through" dirt to keep the power flowing to the locos. Relco is one of the most common brands.
Many people swear by them, some people claim they cause excessive pitting of wheels. In addition, the sensing circuitry to "trigger" these units into operation is incompatible with DCC.

Here are some comments from real people with real experience (with their permission):

Helmut Paule: [ This e-mail address is being protected from spambots. You need JavaScript enabled to view it ]

My five cents of running "Z" , four years. I have Relco hooked up to the Track. except on my ICE Track, where I converted the Lights in 4 Cars for better brightness and found the Relco not efficient anymore because of the Lights in the Car's.
I use the 8802 or 88021 at least once a Month or if I think (believe) its nesseseary to do so twice. Every Direction twice on my Layout. Then I use the Aztec Track Cleaner Car also about 5 to 6 Times around the Tracks. If I come back from a Show, (usually outdoors) I clean up with Alcohol the Tracks, using a Leather (shammy) or Joergers Handle with Felt (dipped in Alcohol), specially on Tracks with Cathenary!

8802 - will clean dust and grit from track, does not clean grime or grease, wheels need to be cleaned manually, clean the Wheels of the 8802 or 88021 after every run

The Aztec has a rolling Cleaning wheel, I need to clean the cleaning wheel after 2 - 3 runs on my Layout, it depends also how dirty it is or looks, I use Alcohol, then water and soap on the rolling Wheel to clean.

(Question from Greg): "There is a new marklin cleaning car that is coming, I was confused, it seems to be 86501 see: http://www.creativelement.com/z/articles/pictures/86501.jpg
have you ever seen it? any idea how it works?" (Reply from Helmut):This is the Joerger cleaning Car made by Maerklin, with a Felt attached to clean the Tracks. seen only Pictures!

I got the HUG cleaning Car (an German Club Car), tried it, but was not impressed.

Daniel Baechtold, when asked about the Marklin track cleaning loco:

I do own such a railbus track cleaner. I was about 12 years old when I bought it in a shop, and only because the seller told me, that I had to have it (I was young and unexpirienced). Well, it NEVER worked. My track was dirty if I used the 8802 or not. Nowadays I prefer to clean the track by hand and I'm running the whole time a track clean boxcar from aztec. (Link to Daniel's web site)

David Karp has a great writeup on the subject also.