In areas around loco sheds etc., the track rails are often concreted in and ties are not visible.
Using 3 rail track presents a problem in that the center studs need to protrude though anything that is used to fill in-between the running rails.
I recently tried using some air-drying modelling clay and it turned out to be pretty easy to do...
I was able to simply rub it into the track area with my fingers...
Since it is not very wet I do not think it will cause a rust problem with the center studs.
Once dry it can be painted to look like dirty concrete.
Once the paint is dry, any clay or paint is cleaned from the tops of the center contacts.
I used some clay called DAS