Welcome to the forums, Ed!
The A&O uses Micore instead of Homasote. They look similar, but they are not at all the same. Homasote is made from recycled paper. It absorbs water, swells, and isn’t dimensionally stable.
Micore, on the other hand, is made of mineral fiber. It is commonly found inside the soft partition walls used to divide office cubicles, and as insulation for kilns. When water is applied it doesn’t swell. It cuts very easily with no dust using just a simple knife blade. The main drawback is that it doesn’t grab spikes very tightly when hand-laying track.
If I recall David found a supplier in Denver that carried 4x8 sheets.
Looking forward to seeing you when op sessions resume! Bob