[Gary in WA]

Backer board has a high permeability rating = 2.8 perm. Plywood, 3/8" = 0.75 B.b will absorb the humidity moisture like a sponge and store it as concrete is a water reservoir material. If you seal either product, the tracks will stay rust free. Plywood of choice for the loads. Use the correct plywood for the load, remember to use the reduction for single support vs. multiple supports. (Walking on, etc.) http://www.trioforest.com/pdf/Load-Span_Tables.pdf Use a vapor barrier primer- or three coats of exterior oil house paint, as Ron and others said: http://www.panhandleinsulation.com/buildingmaterials.html
Try some sound board sheathing under the ply to help deaden the noise echo if the layout is elevated. Curtains on the walls help too. Keep some large Rubbermaid containers (with lids) nearby if your location is subject to frequent power outages of any duration. Pop the engines into a box along with a dryzair packet or bowl to keep them dry if needed. Larger zip-lock bags work also, suck the air out with a straw before complete closing.

Be safe, Gary