Yeah I understand the rail pass is only available to international travellers. I've asked one travel agent for a quote and its substantially more than buying it direct online.

If I do buy it online and plan to pick it up presumably I can just do so from my first depature point, LAX, without any problem?

The cost for the pass over here is £296, about $554. I've costed it out on Amtrak as $511, with student discout. This initially doesn't look like a huge saving but given the $511 also includes a sleeper roomette between SEA and CHI it makes it quite worthwhile!