Question about yard tracks

[MasonJar]

I do plan to add some airbrushed grease and oil along the outside of the rails

On some layouts,especially those running diesels I noticed an oily streak in between the tracks

And there you have another difference in era... The well lubricated, open running gear of steam engines, and grease-soaked friction-bearing trucks of the steam era tended to deposit gunk along or outside the rails. Diesels, with many more "inboard" moving parts, tend to leave a track down the centerline...

Andrew