After you make it through the security barrier and on to your airplane, you might want to use your computer during your voyage. On a short flight, you may be able to use the computer's internal battery, but if you're traveling from coast to coast or overseas, the battery probably won't last as long as your flight. The security issues aren't as extreme when you travel by train, but you might face many of the same short-battery-life problems on an extended rail trip.
Some airliners and passenger trains have power outlets for personal computers and other electronic devices at many seats, especially in premium class sections. If you can find one of these outlets, you can use your laptop for the entire duration of your trip. Of course, you're still obligated to cooperate with requests from the flight crew to shut down electronic equipment during take-off and landings. When you reserve your seat, ask about access to a power outlet.
To avoid conflicts with other passengers for access to a limited number of outlets, carry a three-way cube tap outlet adapter.
If you can't use AC power at your seat, carry a spare battery. Some recent laptops can accept extra-large batteries with longer charge time than the ones supplied with the computer; they're somewhat heavier, but they might be ideal for a longer trip.
Other portable batteries connect to the computer's external power input connector. Some are less than an inch thick, so they can fit in the bag next to the computer itself. Many of these external laptop batteries are quite expensive, but they can run your computer for an additional three hours or more. Before you buy an external battery, make sure it provides the correct output voltage for your computer.
Computer security doesn't stop when you pass your laptop through the X-ray machine. Don't leave your computer in plain sight when you leave your seat, or it might not be there when you return. If your computer is stolen, notify a flight attendant or conductor immediately.