Alcohol, or some kind of light oily cleaning fluid might work best. No stuff that will dissolve paint, though.. Also regular cleaning stuff like "Ajax", dishwashing liquid and such will probably work well on the gunk... This means manually scrubbing the tracks with a rag or such.

One unusual tip on the MML some years ago, was to put the tracks into a dishwashing machine and wash them there... But since the M rails are hollow, water might not be totally drained from inside the rails and they may rust from the inside...

I myself once manually dishwashed my old M-rails with a brush, as you normally do dishes, then put some of the tracks to dry "naturally", and put some into my washing dryer cabinet at max temperature... The ones that were "naturally" dried rusted, but the ones in the cabinet seemed ok...

If I was in the situation to clean M-tracks again, I would not go for the methods that involve water en masse, unless you can dry them quickly. The main thing to watch out for is to keep the inside of the hollow rails dry... Once you have got water inside those, only heat can dry it out... And you better do it quckly, since the rusting starts within hours...