What did you do to your trains today?

You're welcome. I did not (specifically) refer to a Q-tip, but cotton pads/balls dipped in multiple shades of grey, white and/or brown, green, etc.

Maybe I should've found the precise name to define what I meant back then. Terminology/cultural differences aside, it did help improve the look of the ballast even though it's only one extra colour. Optionally, it would also look good for the wood highlight on the sleepers.
 

They really do look very good, but I am curious about your decision to leave unpainted those parts of the sleepers/ties that are outside the rails?

They didn't "decide" to leave the grooves unpainted. That's where the rolling stock's wheels run and paint would most likely rub off (unlike in the middle of the track).