Another vote for Birch plywood...
BTW, fastening the wall brackets firmly is paramount. Fastening the sub roadbed (birch ply) to the wall brackets is next most important.
However, fastening the track to the roadbed is, IMHO, a matter of keeping the track from shifting side-to-side.....because gravity does a pretty good job of keeping things 'down'. IOW, don't fret too much about a lot of thread power in keeping the track in place.
Now then, I would strongly suggest the addition of a safety wire/railing on the outside of the shelf to retain locos and cars that would like to be Tootle....off the rails, picking daisies. If there's a chance a client or co-worker could find themselves under attack from falling trains, your insurance company would probably appreciate this addition. It need not be celebrated. That is to say a run of wire about a half inch above rail height should be adequate and not obstructing the view of the trains that much. On the commercial installations I've done, I found that the eye lag screws...
...used mainly in the hanging of suspended ceiling rails/panels are a very handy, inexpensive, quick way of providing support for a safety wire....for which I use a very thin steel cable, such as included in picture hanging kits but certainly available in bulk, cut-to-length at larger hardware, home improvement stores.
Other than that....have fun.
Oh yes, another lesson from the School of Hard Knocks....keep your train controls out of reach of curious customers. There's 'hidden', and there's 'locked'. One client found that 'hidden' doesn't mean much to determined Easter egg hunters and masters of playscapes
....or bigger kids who never were smacked for inappropriate curiosity!
FWIW, of course...
KD
