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.

[Super]

What do the magnetic couplings look like once they are removed?

somewhat like the tomy magnetic couplings

[Super]

You mean like one has a hole and one has a t-bar for installation?

Rather than buy these trains just to harvest the magnetic couplings how about these custom designed ones?

I've designed them so that you can glue 10mm diameter circular magnets to the disc part and this will then swivel left and right in the frame, thus enabling the coaches to take corners better without straining the magnetic connection too much.

So far I've made 3 variations on the design with 3 different Tomy connection types which should cover most engines and rolling stock (see pictures below);

Type 1




Type 2


Type 3

I printed a prototype of the 'type 01' connector, this came out ok apart from the pin on the top of the Tomy end, this is possibly my fault when I removed it from the bed and stripping off the support material.  It was also only printed in draft mode (0.2mm) resolution so final versions would be far more accurate.  Anyway so you can see it here's a small video of it, I'm also not sure about the height of the unit, I may have to try another design, but at least it did do one thing I wanted, print in a single piece and still have the swivel I was looking for

You mean like one has a hole and one has a t-bar for installation?

yes. unfortunately my rolling stocks are stuffed inside the storage locker.  I can't post a photo of the coupling system.

Rather than buy these trains just to harvest the magnetic couplings how about these custom designed ones?

I've designed them so that you can glue 10mm diameter circular magnets to the disc part and this will then swivel left and right in the frame, thus enabling the coaches to take corners better without straining the magnetic connection too much.

So far I've made 3 variations on the design with 3 different Tomy connection types which should cover most engines and rolling stock (see pictures below);

Type 1




Type 2


Type 3

I printed a prototype of the 'type 01' connector, this came out ok apart from the pin on the top of the Tomy end, this is possibly my fault when I removed it from the bed and stripping off the support material.  It was also only printed in draft mode (0.2mm) resolution so final versions would be far more accurate.  Anyway so you can see it here's a small video of it, I'm also not sure about the height of the unit, I may have to try another design, but at least it did do one thing I wanted, print in a single piece and still have the swivel I was looking for

type 2 and type 3 seem to work the best.  My problem is where can I find a magnet that fit exactly in that slot?  If you provide a magnet glued to your 3D printed coupler, it would be a lot better.

I could provide them with magnets fitted, but if shipping to the USA it would add to the weight, the ones I found were on Amazon UK 10mm X 2mm I can post the link if anyone is interested. I've since printed a prototype of type 2 and it came out superbly :-)

This photo basically Suma up all the imaginarium power rail series. Sadly this line doesn't have a lot of products. The engines are too high to clear Tomy tunnel and bridges.