Maintaining good electrical continuity between model railroad rolling stock wheels and rails is a topic that has consumed a great deal of printers ink. Of course, more recently computer space has supplanted much of the ink.
No matter how well wheels and rails are cleaned, over time an extremely thin layer of "dirt" builds up. This is a combination of oxidized metal caused by micro-arcs between metal wheels and rails and dust or other debris that naturally settles on everything in our homes.
My blog post of September 11, 2020 titled "Never Clean Track Again" explained how a very thin layer of graphite can be applied very sparingly to the rails. In my experience, this dramatically improves electrical conductivity and significantly lengthens the length of time between cleanings.
My blog post of December 21, 2020 titled "Flicker-free Car Lighting" explained how I installed LED lighting in several passenger cars and, through the use of supercapacitors that hold a significant electrical charge, prevented the flickering of the lights when electrical continuity is momentarily disrupted. That design required the use of electrical wipers that rub on the metal axles of the cars so electrical power can be fed from the rails, through the wheels and then to the circuit inside the car. While the electrical circuit did its job and the graphite on the rails also did its job, I had problems maintaining good conductivity between the axles and the wipers - graphite didn't work well for this, I suspect because the axles are very highly polished metal.
Several months ago I read an article that presented another idea of how to maintain electrical continuity. If I could remember the article I would give credit to the author but at the moment I cannot remember where I read this. The suggestion was the use of a special conductive grease named NO-OX-ID made by Sanchem, Inc. According to its website, Sanchem is based in Chicago and makes a full range of rust-prevention products and has been doing so since 1918. They make rust prevention products for the automotive industry as well as for pipelines, pickling equipment, dams, canal locks, etc. After reading about NO-OX-ID I placed an order with Amazon. Here are the details:
I first used a toothpick to apply NO-OX-ID to the axles of my lighted passenger cars. Like magic, the electrical continuity was so good that it was as if I had hard-wired the circuit in the cars directly to the rails. That was three months ago and the conductivity is as good now as it was then.
I also applied a VERY light coating of NO-OX-ID to a few centimetres of rail at a few locations around my model railroad. The passing of rolling stock gradually distributes it around the layout. Before applying, it is a good idea to do a good cleaning of the rails to remove accumulated dirt. Again, I did my first application three months ago. The results are truly remarkable. Even locomotives that do not have "stay alive" circuits in them operate as if they do.