xkcd

Yeah, but road trips can be expensive. Suppose you want to go from Harrisburg PA to Rockford IL with 2 adults and 1 teenager from November 15 to 22.

• By car that's about 1500 miles. An average car gets 21 mpg, so that's about 71 gallons. Gas is around $3.5 per gallon, so the trip costs about $250 in gas. You'll need a hotel. I picked a random one in Ohio. $110 for the way up, $185 for the way back. I guess that's a Thanksgiving price hike. $545 total.
• By train, let's say Amtrak because that came up first. $438 up, and that includes boarding a train at midnight and sleeping on the train, and then riding a bus from Chicago to Rockford for 2 hours. $483 back down, and this time when you sleep on the train you have to wake up by 5 AM to get off. Also this is coach class, and those seats aren't great for sleeping. At least you don't need a hotel. $921 total.
• By plane, it's $650 round trip, simple as, but you have to leave at 6 AM on the way up and 5 AM on the way back. It can cost $200 more to get a more convenient time, but let's assume you're going for economy alone. $650 total.

That's not accounting for food prices along the way. That could bring the car ride up to the same price as the plane if you don't pack food, but if you're spending extra on convenience there, you're probably willing to spend extra for convenience on the plane too.

So it's probably safe to say that, for this group, the car saves about $100 per year, but helping to protect the environment is worth that price. On the other hand, there's something to be said for the flexibility and ease of planning on a car. For a bigger family, cars would be a way better option, and for a family without kids or a lone traveler, planes are the way better option. Trains are right out.