any magnet coupling mod for Trackmaster?

Correct.  The product line is called "Imaginarium Express Power Rails"  See below 



Instead of retaining the magnetic coupling, I converted them into tomy style coupling.  See video below



I don't recommend people to retrofit every rolling stock out there especially the ones with moving parts like that mixer rolling stock.  I had a hard time to put it back together as if nothing happen due to numerous moving parts inside the chassis.