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Many people will tell you that Carrera at 3 7/8" is the minimum for running the big scale. Others will tell you that Ninco at a hair over 3 1/2" is adequate. I will say that there is nothing like getting out the old ruler and measuring the cars you want to run and decide what you need. If you are running closed fendered cars, they will run fine rubbing doors all of the way around the track. Open wheelers are another story. They need room to roam. For closed fenders, measure the track width of the biggest cars you want to run and add 1/8" or so. That will allow you to pass with those cars even if they do some door rubbing. As for gutter, on the inside it's about 1/2 the width dimension plus a little. If there is a drop off add more, or a curb to keep the inside rear tire to track off the edge when going slow. For the outside, take your longest car, cock the tail end out to the point that you decide will beyond any advantage one might get from rubbin' the bumper around the rail and that is good enough. If you make the gutters plenty big, you can make them appear smaller by simply painting the scenery into the roadway to narrow the visual look. After all real racers cut the corners of the race track all of the time don't they? You can do the same painting trick on the outside.

I used the reverse side of a previously used sheet of mdf to create the corkscrew on my most recent circuit - Woodland Trace Raceway. I used it because the sheet is only 3/8" thick and I needed that to get the flex I wanted for the corkscrew. There are 1/4" deep slots on both sides of this sheet and I added additional kerfs to the underside to get it to flex the way I wanted. The track has been in operation now for almost 3 years and I've had no problems with it. Of course, the slots are not congruent. There are a few places where the upper slots cross the lower slots creating openings but no problems with that!

I used full 3/4" for my track so it's heavy and solid. But there are a few places where there are slots on both sides because I started and then changed my mind, and just flipped the sheet. No problems at all.
I suppose you could have a problem but you would have to rout in the same spot and a pretty long distance before it would cause you grief. Just passing over and through you will get the openings FC referred to but that won't hurt MDF at all.