I am planning the route for our driving trip from Italy to Germany through Switzerland in late July, and online searches are recommending that we transit via two different "car carrier" trains: the Iselle IT-Brig CH "Simplon" train, and then the following day, the Loetschberg tunnel, also a car-train.

Questions for those with more local experience.
1. Are these trains first-come, first-serve? Is a timed reservation necessary, as often for a ferry?
2. On weekdays in July, what would be the typical wait for the next train transport?
3. Approximately what are the costs? Is cash necessary CHF ?
4. Our rental car will be a standard Hertz car, possibly a SUV -- any size or registration restrictions to be aware of ?