A road mat fastens onto the beam and makes it wider(I'm sure a coach will know how wide). Sometimes they'll put them on the floor so the girls can work a new skill.
It makes it 8 inches I believe. And a lot softer. I freaking love those things. I can't even adequately express my love for them, but it is vast.
We also call them railroads. They have a dotted line running down the center.
I did a RO dismount in L8, but then I randomly switched to front dismounts, because everyone else in the group was training those and things were always set up for it.
If you have a CW back tuck dismount and a RO on the beam then you are headed in the right direction. I would make sure the RO is "over the top" (as opposed to around the side) and shows adequate flight (air time from hands to feet). No piking down.
Then we usually trained it on floor and off a railroad onto a mat on the floor...my coaches wanted to see the ability to do standing RO-BT on floor/at the surface height of whatever we were ROing off. I think that's a reasonable criteria but perhaps not necessary. Then we usually put a crash mat behind the high beam and did RO-set with the railroad on high beam. Then RO tuck (onto the crash mat). Then we took away the railroad, and used a mat over the beam. Then a carpet. Then nothing. Then we did it without the crash mat onto a couple 8 inchers, and then finally competition landings. We usually used much higher mats and the carpet for training though, until it was right before a meet. Now most of the L8-10 girls do BHS dismounts, although there are a couple ROs. We use the same set up and they work on track a lot, doing either standing RO or BHS into tuck, layout, half, full onto a mat stack.