The main reason for the 110 psi is for aux apps like water systems, air doors and toilets. In most situations this is done with main res and not train line but in the rare case of a car that is not set up for main res or has been changed to take air from the train line, the car "could" make an app and the engine would not see it, the valves don't much care since when the car is full service or even in the hole, the cyl is only getting 88 lbs. At the end of the day I have only had one car do this and the car was a dome with air doors, water and septic all off the train line and the engineers all had problems with the car, I did four single car tests all with the same passing grade. It was not until I rode the car and noticed that every time a passenger went into the restroom we had an app that I started looking around. In a horrible case one could argue that several of these cars in one train could cause the train to dump and hold up traffic. As for Graduated release, it works great as long as all the cars are set up for it, if you have a nine car train with one car set up for direct release then all the cars will act like direct release. As for the idea that a freight car cannot be in a passenger train, that is wrong, most Amtrak private cars now have ABDW brakes. ABDW is a freight brake and this is something I had to go over when I got my Airtech card, per CFR 49, I had to understand that passenger cars can and do have freight brakes. Then you have the oddball stuff like the MARC budd coach system, it has a DB valve for service and to muddy up the waters, a #8 vent valve for the EMER valve. In the event you have a passenger car that is in direct or graduated and you want to swap it, the service portion will have a plate that with two ½ inch bolts, when the lettering is upside down, the car is NOT in that application, so if you can read it fast, right-side up, that is what the car is set up for. This is typical on D-22 cars. Then again, it is worth pointing out that passenger car brakes come in a lot of different flavors, D-22, 26-(L)(D)(C)(F), UC, , ABDW, DB, L, and my favorite, KE3.7!
Now for the disclaimer that I give my customers, NEVER ship passenger equipment in Graduated release, always be sure to check and make sure the car is in DIRECT release, freight railroads are NOT trained in passenger car brake systems and WILL NOT notice this and the result could be your car being damaged. And since I pointed out KE3.7, if you come upon a car with KE3.7, do not attempt to repair it yourself, call your favorite car repair company and get your checkbook out and upgrade to 26-C.