One place you can still see retainers in regular use is on the C&TS narrow gauge. They have miles of grades 3-4% on the West side of Cumbres pass. About 12 years ago we road the rr Chama,NM on the West over the pass and down to Antonito, Col. on the East end. The grade is less on the the East side, but being an amateur student of air brakes I noted that the ride started getting a little rough with a lot of slack action, then I noticed the conductor turned up the retainers on the last 3 or 4 cars for a ways which smoothed out the ride. The next day we road East to West in part because I wanted to experience going down a 4% grade. I believe the retainers were turned up on all passenger cars before we rolled off the pass.
The fatal accident that occurred on the White Pass and Yukon a few years back with a work train that ran away was a few years ago was as I recall blamed on improper use or lack of use of retainers turned to the proper position.