Traveling from Sacramento to the Bay area by train. I need to get on the internet and Amtrak still doesn't have a wi-fi that works. What do I purchase to plug into my computer?

You could get a laptop connect card, which lets you connect your laptop wirelessly like a cel phone. Contact a major cel phone provider for this type of service for details and costs.

Its called mobile broadband. There are some offered on a pay as you go basis. Virgin and Verizon I know have them.

Depending on the phone you carry it may also already provide an internet connection. You might want to look there first.