Last time I checked, it was still a lot cheaper to fly than choo-choo. I bet cheap wins.
Money is data, which is why most money is stored and processed by computers these days. The price of something tells us roughly the amount of energy that was consumed in making it, as well as how much pollution was generated. It is easy to determine if product A pollutes more than B. Half an airline ticket is taxes and fees, so there's obviously a whole lot of CO2 emissions taking place indirectly that have nothing to do with flying. A train ticket only covers 10% to 25% of the total cost, with taxes covering the rest. Without train subsidies nobody would ride them as they would be priced out of the market. It doesn't take a rocket scientist to determine which is the biggest polluter. It's not even close.
Force equals mass times acceleration... it takes the same amount of energy to move you from Frankfurt to Belgium regardless of how you travel.