Ok, A disclaimer here, most of my track over the last 40 years has been hand laid. recently i have been using some commercial track components at the club, on hidden track on my home railroad, on the narrow gauge on my home railroad that is on a shelf hanging from the ceiling where hand laying track would be difficult or impossible. there are tricks to installing commercial track that I had forgotten, or never known. Making my own track from scratch, I had thought using flex track and switches would be easy and quick. I was wrong, careful attention to detail is required
If your ties are breaking at the nail holes one of two things is happening; either the hole is too small ( or the nail is two large- take your pick) a track nail should slide easily through the hole in the tie, if it does not, get a thinner nail, or drill the hole out so the nail can be driven into the hole without hard contact on the sides of the hole; or the nail has been driven down too far bowing the tie downward in the middle. The latter can narrow the gauge slightly @ the nail location, or break the tie. I like to leave a couple thousandths slack between the nail head and the tie surface, which allows the switch to float a little , and insures you are not damaging or distorting the tie with the nail . If the nail is long enough to hit the plywood subroadbed, it may be a good idea to drill a pilot hole with a drill bit that is smaller than your track nail, so the nail still grips the wood, but it takes less force to drive in; that makes it easier not to drive the nail all the way flush with the tie, or farther, which might bow the tie downward ore break it.
Hope this helps, try it , and share your results with us here
Bill Nelson