U.S. citizens are required to show photo identification when traveling on a domestic flight or Amtrak train, or when driving within the United States. A driver's license issued by the citizen's state of residence is the most commonly used form of identification, but a passport can be used. Amtrak, for example, requires a valid, government-issued photo ID to be presented when a customer is buying tickets, storing baggage or traveling on a train.
Legal, permanent residents of the United States--also known as green-card holders--are legally obligated to carry their Permanent Resident Card with them at all times. It is also a good idea to carry a second form of identification, such as a state-issued driver's license or a passport from the country of your citizenship. Keep copies of your identity documents stored at home while you are traveling, in case your original documents are lost.
Travelers who are neither U.S. citizens nor U.S. residents must carry government-issued identification for travel on domestic flights, Amtrak trains and when driving in the United States. Examples of acceptable documents include a passport, visa and foreign or U.S. driver's license; foreign nationals in the U.S. on a visa are eligible to receive a U.S. driver's license.
