Chapter Time Sequence

I never realized until now that being a mystery novelist can sometimes mean re-arranging time. I recently spent four hours trying to figure out a proper arrangement for my chapters. You remember in algebra when we had those story questions--Train A leaves Station A at 6:40 heading east on Track A traveling 40 mph and Train B leaves Station B at 10:00 heading west on a parallel Track B, traveling 60 mph. If the stations are 83 miles apart, at what time will the trains cross paths? My book is like that. I've got two groups of people whose actions start five weeks ahead of the first murder, but I cannot reveal their existence until after the third murder happens. So some of the chapters leap backwards in time. Then, when the trains finally cross paths, I have to speed the time up for the two groups that started early, so their actions now parallel the cops/murders. Got that? Neither did I. I thought I had it figured out three times, but each time when I finished re-arranging chapters I would see the flaw in my reasoning. I thought I had solved some of my problems by moving one of my groups into "real" time, but after sleeping on it, I concluded that my reasons for having their thread start way back were good reasons, so I'm going to change that. This explanation doesn't seem to make sense, but I think it will in the novel. Thanks for reading. Joan Sween