The USA Rail Pass (15-day/8 segments/$489; 30-day/12 segments/$669; 45-day/18 segments/$859) covers the entire Amtrak network for the designated period, though you are restricted to a set number of individual journeys. The California Rail Pass buys you seven days’ travel in a 21-day period within that state for $159. Passes can be bought from the Amtrak website (amtrak.com).
The main American airlines offer air passes for visitors who plan to fly a lot within the USA. These must be bought in advance and are often sold with the proviso that you cross the Atlantic with the same airline or group of airlines (such as Star Alliance). Each deal will involve the purchase of a certain number of flights, air miles or coupons. Other plans entitle foreign travellers to discounts on regular US domestic fares, again with the proviso that you buy the ticket before you leave home. Check with the individual airlines to see what they offer and the overall range of prices. However you do it, flying within the USA is only a wise choice for travel in regions where fares are low anyway; flights within Florida, for example, are very expensive.