brass is affecting the adhesion by getting corrodated on the upper surface very fast. Some kind as silver does, too, or untreated Aluminium.
It does not corrodate deeply, because the corrosion-skin prevents oxide to reach deeper strutures.
But for this skin, the brass rails have sometimes a better "grip" than others. Disadvantage: the corrosion-skin does not let current flow.
So u have to clean it or drive (like i do) R/C-accu-operations.
U can roughen a solid brass railwith sandpaper, what brings all three effects fast and good: good traction, good current flow and fast corrosion to make sure it does not work any more 3 or 4 days later.
nicle-plated rails are more slippy, because the skin tends to fill the scraps and stays gloomy and shiny, what always is a "pointer" for good slip.
But the flow of current is well. And it does not corrodate.
Roughen the nicle is dangerous. When u reach your brass-underground, u loose the well current-picking-attribute. For more, Nicle isnt very hard, you will drive down the roughended surface quite fast (i only talk about roughening with a middle-fine sandpaper. Rubbling it up with a deep knurl is -in my eyes- not good.
So it is at yours. Me, i would EVER prefer to have a safe current-picking than pulling 2 cars further.
And one time more: thiel-gleis does nicle-plating with used tracks. No clue here if its worth it.
Greetings
Frank