Some of the "gunk" is actually the oxidation layer on the track. Nickel Silver and tin have oxides that are actually conductive (not as conductive as the metal but not an insulating material either). Tin oxide is dark grey and has a slightly greasy feel to it. This material tends to attract other debris. Smoke fluid (which is usually some kind of mineral oil) and excessive lubrication are the principal "culprits". This "stuff" can cause the traction tires to wear down faster and add even more crud. Since the track is now used for signal communications purposes either directly or indirectly the need to keep track clean has increased dramatically.
Abrasives should be avoided, e.g. sand paper or emery cloth as they can remove protective coatings (like tin plate) and leave debris (particles that can mess up gear trains). Steel wool is also a very, very, very bad idea (the iron fillings will get into anything magnetic and mess up gear trains as well). Any solvent should be checked for compatibility with the materials it will come in contact with and human safety factors like prolonged skin contact, inhalation issues, and/or ignition issues. You want something that doesn't leave residue.
Count me in with the group that uses high percentage Isopropyl alcohol (90%). Splash some on an old tee shirt "rag", wipe down and then done. This is not the same as the usual 70% that's typically found in the medicine chest. 99% stuff is available but you can watch it evaporate from the bottle and it's pretty good at pulling water out of the air so it doesn't stay 99% very long. It's not worth the extra cost plus it's really flammable.
