Following Kati's excellent post, Trains vs. Planes in the Emission Reduction Race, here is a list of reasons to motivate YOU to take the train on your next trip. No matter how fast they are, planes (and airports) are a major pain and there is no way around that.

1. Flexibility! There is no beating trains for flexibility. Missed one? No problem! Just take the next one. You don't have to wait (sometimes for days) for the next available seat. Tickets are generally open, which means you can just buy and board, or use it at any time of your convenience. You don't have to book your trip months in advance. You can get on and off at any intermediate stop and have lunch with your high-school friend.
2. No waiting. Remember that on top of the flight time, you need to be in the airport 2 hours in advance, in order to suffer all kinds of indignities security checks. Wait 1 hour for your luggage. Add another hour (at least) to get to the first airport, then another one from. And nowadays, some trains go as fast as 300 km/h. Still think planes are faster?
3. Location. Trains stations are in the city center. Not in the middle of the woods somewhere 2 hours from where you have to be. The minute you are off the train, you are right where the action is.
4. No nuisance. The fact is, taking a plane is full of little nuisances. You never know if you might get stuck in transit because the weather is bad or your flight was overbooked. They'll take away your expensive perfume. They'll take away your (reusable!) water bottle. They make you take off your shoes. And the risk of a cavity search will always be in the back of your mind.
5. Technology friendly. You can use your laptop and not worry about causing a tragedy. You can catch up on your phonecalls. (And your mom says you never call!) You can even check your email.
6. More comfort. Plenty of leg-room, walking around and even a restaurant car. Trains are even better for your health, since you are not at risk of suffering from Economy Class Syndrome and heart attacks from sudden onsets of turbulence. And if you want to go all out, try a sleeping car. Those are an experience in and of themselves.
7. Competitive price. Rail prices are getting increasingly more accessible, a trend that is set to continue with the ever rising fuel prices and various taxes and surcharges for air travel. Not to mention you don't have to pay for expensive shuttles/taxis to get to your final destination (The transfer from Stansted to London is more than some Ryanair flights!).
8. Pet friendly. If you have pets, this is a BIG plus! A train trip is a lot easier to endure for a pet, as there are no sudden pressure and oxygen changes which are very stressful for animals. If you have a bigger dog, you don't have to stuff it in a box in the cargo hall, abandoned and freezing. You won't have to sedate your pet. And last but not least, pets can take the train for free!
9. Enjoy the Journey! For the more romantically inclined, the poetic appeal of a long train journey is hard to surpass. To actually experience the distance, the views and the (sometimes radical) change of scenery makes you appreciate the journey itself so much more.
10. Guilt-free! No need to offset your guilty eco-conscience by paying someone to plant trees on your behalf.

And of course, lets not forget the environment ;) But you knew that already. You can find some other good reasons here and here.