As a follow-up question (sorry to spoil the fun)...
I have noticed that the colour of the rusting varies considerably - My favourite example is on the Kyle line at Garve, when the line alongside the road appears to glow orange. What are the variables?
Amount of traffic (and therefore dirt affecting the oxidisation process)?
Amount of moisture in the air?
Different alloys?
Could be all three.
If more traffic is causing more dirt/dust to coat the surface, that would at least make it appear different because a darker substance is present, a situation which would be the opposite in many areas of central Australia! Dirt/dust could also protect the surface and lower the rate of oxidation.
More moisture could increase the rate of oxidation, but how you get to the point of having more moisture present does matter.
If the moisture comes from more extreme temperature ranges over a daily cycle causing more dew to condense it won't make much difference. Lower temperatures slow down chemical reactions, and because pure water is a poor electrical conductor dew is not going to accelerate oxidation compared to normal rates.
If the moisture on the track is caused by mists or sprays of salty seawater on a line near the coast you'll get perfect conditions for oxidisation thanks to the electrical conductivity of salty water.
Differences in the chemical composition of the rail could be an issue in changing the appearance of a rail. Steel with more carbon might appear as a darker brown because it's a mixture of orange rust and black carbon particles.
With your example of observing the side of a rail on the Kyle Line, I would suspect it's simply that it's not disturbed much and that the low usage of the line lengthens the time between rails being replaced so a nice uniform layer has time to form. Lighting could make a difference in how you observe it as well, especially if there is dew on the rail when you see it as well.
The thing many people forget is that rust on the surface of a solid object like a rail or an OHLE pole is not a bad thing. It protects the rest of the metal underneath from corroding, and if it was scrubbed off you would merely start the whole process from scratch but with less of the pole/rail remaining than before.