Personally, I went with used aluminum shelving and it was significantly cheaper than buying new. I found it on a train auction several years ago.
I like it, because it disappears on my walls and one only sees the trains. It is also nice, as one can get long trains on the shelves. I have a 36-foot long stretch of shelving along this wall (the portion that extends over the layout). The upper shelving is ?? (small size) to O gauge and the lower shelving is O to Standard Gauge.
Most of my shelving is spaced for O gauge, as that is what I primarily collect, but I do have an area for Standard Gauge trains
Here is some of the excess O to Standard gauge shelving that is waiting to be installed.
The gap between the lower shelving and upper shelving serves two purposes. First and foremost, my electric wiring runs in the gap between the levels, as this is in a basement and I allowed for flooding and did not run the wiring along the floor level. Second, it allows for extra Standard Gauge trains, should I fill the small area that is spaced for standard gauge, and it allows for some accessories.
As for attaching it to the walls, I used hex head screws that are made for attaching steel siding to pole buildings. I was able to buy boxes of these screws in bulk at Menards and they have a rubber grommet on them, so they snug up nicely against the shelving and are easy to install with a drill and hex head driver.
Another advantage to this shelving is that one can space the shelves closer together as there is no need for space for thick shelves and the brackets that hold them up.
NWL