Depends who you mean by "we". Seems there are regional shortages [1], but the US only hits supply constraints occasionally.

[1] https://www.smithsonianmag.com/science-nature/world-facing-g...

Also discussed on HN:

https://hackernews.hn/item?id=15739917

There is an abundance of sand away from the construction sites but transportation is too expensive so regional suppliers are preferred.

Though it seems like it might be easier to ship sand for pre-stressed concrete crossties, given that all the materials must necessarily be connected to the rail network.

Also, I wonder why concrete crossties don't use neoprene bearing pads under the rails to reduce the abrasion problem.

Are concrete crossties pre-stressed? It makes sense now I think about it, but I'm just surprised more effort than "pour concrete into a mold" goes into such an unassuming structural member.

I'd guess that poured concrete is not cost-competitive with timber crossties, and pre-stressing is the only way to make them cheaply enough to compete.