I bought the Miniart track kit, but then I got to wonder how railroad tracks are actually maintained. Obviously disused ones are rusty, and ones that are in daily use are rusty but with shiny surfaces on the tops. I assume that these rails are polished once in a while. So, what happens as railroad tracks age?

Unused tracks just rust, wooden sleepers rot and crack.
Used tracks get worn down on the top and the side where the wheel flange rubs until they reach a stage where the track needs to be re-railed, sleepers on a used track will have oil and grease deposits on them - sometimes one of them may catch fire and when that happens the crew of the next train along will have to make use of the cabin fire extinguisher to put them out.

Edit: The only polishing that happens is when the trains run over them, they do get re-profiled when required by a track grinding machine.

https://m.youtube.com/watch?v=Kh3_QcnCamE

This video may give you an impression of nowadays methods in the USA.
However: in those days when labour was less expensive than material, tracks were divided in distances where a RR employee would go up and down the tracks (sometimes with a small hamer) to visually check his part. With the hammer he could knock and hear of tracks were e.g. cracked or even broken.
There is a Russian(?) modelling company which not too long ago made some of these men.
Sorry, but I don't know the brand, so, please, hopefully another moddeler can help you here.

The state of track can vary so much even in short distance. The best thing to do is look at some online images of track. Generally they will rust to a dark brown but then lubricant spills from axle boxes, brake shoe dust etc etc all add to the mix. Of course you may be modelling a section of track that has had a new piece fitted, so that will have completely different weathering. The tops only remain shiny if they are run over a fair amount. Even lightly used track will display some new rust in the imperfections on the rail top... like i say, images off the web.

A used track will be shiny on top and depending on age it will be either a light orange-red rust colour (fresh rust) or a dark brown-black rust on the sides.

A disused track will have started rusting on the top and the sides will be the same as for a used track.

The colour of the rust depends, at least mostly, on how time the rusting has been going on. The orange-red rust will form rather quickly. It will also depend on how much oil/grease that is deposited on the track be passing trains. Steam engines spilled a lot but modern rolling stock will also spill oil/grease sometimes and the wheels will spread the oil film over long distances.

/ Robin

The brand name is Armor35!
You may find the track inspectors on www.armor35.ru.