To me, it looks as though the floor in the garage was poured without the use of wire mesh or re-bar. I could be wrong, though. It looks like the garage is settling. You could fill the cracks, but that's like putting a Band-Aid over a gaping wound. You may want to have a professional look at that floor. Putting a model railroad on top of it would be an exercise in frustration. You would be constantly dealing with uneven track-work, and the issues that go with it. I don't mean to be a downer, but you really need to get that floor dealt with.