Any tips on how to treat these bad boys?
J

You could use burnishing fluid,like from AK.It does require thorough cleaning of the tracks and the fluid can be tricky,but when done right,it looks good.What I have been doing is priming,painting,and weathering with regular paints,washes,and Pigments.I do sand it off the high points to expose some bare metal.

There is (or was) a product called "Blacken-it" (is that politically correct??) that chemically blackened metals, mostly used by RR modelers to blacken the nickel plated tracks. If it is no longer available, I know there is a similar product. Check model RR shops.
On Friul tracks the product gave a blotchy rusty brownish patina which could be sanded off high points to bare metal. Lots of modelers here have used it.

https://www.solidscale.de/solid-scale/solidtrack/

that one is great, I used it many times.

Just get some perma blue from walmart or a sporting goods store clean your tracks with distilled viniger dry then soak till they look like you want them not over 30 min then scrub again with a tooth brush and water dry then scrub agin ifwith a dry tooth brush if needed then weather with paint or pigments just finihed a set worked good for me

I mix table salt(sodium chlorate) and bluestone(copper sulphate) in 1:1 ratio, add some water, stirr and let the mixture rest for 10 min.
Then submerge the links and shake(well sealed container is a must).
I found out 10 to 15 min. "bath" time are enough for my tracks, but I usually pick a track link from time to time to check the colour.
Once happy with it, I drain the solution and wash thoroughly the tracks under running tap water.

Whatever method or product you use, you should also use a wetting agent, or agitate well during the process. Any air bubbles clinging to the tracks will prevent the chemical reaction and leave voids or blank areas in the finish.

Thanks everyone for all the very useful tips and advice!
J