Per earlier comments, it's not just the rust colour on the steel rail that varies, but also 'the other stuff' that arrives on it, brake dust and 'other' from the train operation, and wind blown dust from the local soil or rock.
In your shoes, what I would do is go on Google streetview and look at areas in your chosen location where rail and road are near each other, and any grade crossings you can find. Also try Youtube for local train enthusiast operational videos. (I was well surprised when I got to see North American tracks well outside the urban industrialised zones in the mid West. Grey and dusty rather than the predominant iron oxide tones seen in the UK.)
Thanks for your reply, and the information & suggestions.
Yes, I'm aware of 'the other stuff' that you mention: Hence my mention of 'The Elements' in various locations (that I mentioned in another Post).
I have actually been looking at Street View on Google Earth; but it's hard to get Close Ups. Also, with different Monitor Settings, Colours can be deceiving. Also, when I have my Laptop plugged into the Tv, the Tv Colours are somewhat Brighter; and it's hard to tell which is more correct.
I've also been looking at YouTube, but once again, the same situation applies.
Anyway, I saw that someone suggested Tamiya Red Brown XF-64. I have a bottle of that, which belonged to my late brother: He was building a HO of a location in NSW Australia, and was using it for the Rails, because it was about the right colour. I tried it on a section of Track that isn't so easily seen, and it doesn't look too bad: So, I might use that on other parts of the Layout where Natural Light makes it more visible, and see what it looks like.