Can someone synopsize the MR argument why “molecular polarity” per se has anything to do with residue formation? There are some peripheral issues with hydrocarbon composition, relative evaporation, and perhaps ‘adhesion’ to metal surfaces, but these are not easily addressed by the chemical difference between ‘polar’ and ‘nonpolar’, so I suspect there is some metal reactivity thing (perhaps similar to why there is a ‘metallic’ smell to some alloys) involved, perhaps with the nickel as catalyst.
Think of the ‘dielectric constant’ thing as being a layer of insulator between two conductors. At a certain voltage difference this insulator will break down and conduct electricity, but the adjacent rail and contact wheel or shoe will act to an extent as ‘plates in a capacitor’ so when the breakdown occurs there’s a small rush of current and this creates the microarcing effect. So if you want a ‘protective layer’ of some kind, you’ll be interested in its ‘net’ dielectric strength independent of layer thickness in practice … which involves a little more tinker than just using a compound dielectric constant out of a handbook or something, but is precisely the sort of thing that an organization like MR could work out and publish.
Obviously the lower the vapor pressure of something wiped on the rails, the longer its film will last. The idea of WD40 is that its solvents clean and remove the contaminants, then the light-oil fraction forms the ‘barrier layer’ that (due significantly to its polarity, now in a meaningful chemical context) gives the water-repelling action that was the original point of the WD40 product.
It might be fun for someone to instrument a Dapol or other car to measure the practical dielectric constant of TOR film over a range of track, perhaps at frequent intervals using a controlled voltage ramp to a clean wheel or contact means of applied or rolling (not sliding) ty