As a guest engineer, I ran a D&S train down form Silverton for a while. They use the straight air for just about everything. Automatic is not touched. I believe they had a rule stating the automatic air and straight air are not to be used together.

That makes sense in their case as the straight air is fed into the brake cylinder through the port in the triple valve where the retainer is attached. The only way the straight air can get into the brake cylinders is when the automatic air is in release, and that port is in communication with the brake cylinder. If you made an automatic set, then put straight air on top of that, nothing would happen as the straight air would go into the triple valve and run into a dead end. You'd see pressure in to pipe, but it would not get to the brake cylinders.

I never saw it done, but you could use the straight air as a continuously variable retainer system. You could make an automatic set, lap the straight air vale to hold air in the straight air line, then release the automatic. The air would exhaust through the retainer port and into the straight air line. It would take a bit to get enough air in the straight air line to create a retaining effect, but then you could make an automatic set, release it, then by feathering the straight air valve, ride the release until you wanted some more air, then start again.

But, they probably have rules against that too....