Welcome to the Forum! I think I've seen a post where you sign off as Nato -we'll call you that unless you tell us otherwise.
I went for a concrete foundation and breeze blocks (which is what they are called here; maybe aerated or cinder blocks with you?) which are light in weight and easy to cut. That required quite a bit of physical work, but I'm young(ish) and also tight-fisted, so mixed all the concrete and cement that I needed by hand. I pity the person who will have to remove it when I'm gone...
Timber is light in weight, easy to handle, quick to build -in many ways the perfect material. The downside is that it can only ever be considered temporary in UK climates 10-15 years at most, I would suggest. I don't know how NJ compares to here, climate-wise.
Whatever you choose, enjoy the process of building your own railway, and be aware that it can be addictive!