Illegal immigrants in the United States need planes and trains. Do they need anything besides passports? I hope you can elaborate, thank you!

When you fly and train in America, they just check your documents, whether you are an illegal immigrant or not. Just look at the photos of yourself and your passport. They are not immigration police, and local Americans don't know whether immigration is legal or illegal. In short, you can either provide him with a passport or a driver's license.