Another lovely shot of the coach. I cannot find the ninth walking groom either.

Every time I study the good pics of the coach in procession I wonder just what that team is trained to do in an emergency.
Each walking groom must control a horse of course, if he can!
I cannot imagine that the rest are required to throw themselves in front of the wheels to stop the coach. It could not be allowed to just roll on.
We can see a brake on the back wheels. So there must be a lever or two to throw. Electric buttons maybe?