Sometimes, a good TV zooms ahead, instant and unstoppable. Just ask Craig Bartlett, creator of the amiable new �Dinosaur Train� on PBS.

�When my son Matt was 3 or 4, he � had piles of dinosaurs and piles of trains,� Bartlett said. �I thought, 'If I made a show combining dinosaurs and trains, I'd have all the 4-year-olds.'�

So �Dinosaur Train� roared ahead � or should have. Instead, it chugged along; Matt is now in college and no longer in its target market. That �says something about the development process,� Bartlett said.

Maybe the delay was a good thing. In the 15 years since Bartlett thought of the show:

� He had a successful run as creator and producer of Nickelodeon's �Hey, Arnold.�

� PBS became a prime pre-school stop. It will launch �Dinosaur Train� with a four-show Labor Day marathon (starting at 7 a.m. on KHET), then run it each weekday.

� The Jim Henson Company returned to PBS, where the late Henson sparked �Sesame Street� 40 years ago. The marathon will be hosted by another Henson/PBS character, Sid the Science Kid.

� Computer-generated animation has improved. The Hensons have gone from puppets to traditional cartoons to CG. This show �is really a little bit too grand in scope� to do without computers, said Lisa Henson, co-CEO of the company her father started.

� And information about dinosaurs keeps changing. �It's a really cool, living science,� Bartlett said.

Scott Sampson, the show's resident dino-expert, offered an example: �'Jurassic Park' portrayed a velociraptor without feathers. A few years later, we find feathered dinosaurs in China. It turns out, probably all of those raptor dinosaurs were feathered.�

For consistency, director Steven Spielberg kept them featherless in his sequels.
Sampson has an alternative solution: Each �Dinosaur Train� includes two 11-minute stories, plus some live-action things he does with kids; if there are new discoveries, he can talk about them.

Then again, this is no documentary. Dinosaurs didn't ride trains, you know.

The story has a baby tyrannosaurus rex being adopted by a pteranodon family. He plays with creatures that a real carnivore would devour. �He's not going to eat them, because he loves them,� Bartlett said.

Still, there are basic truths here. �Kids are fascinated by huge situations,� Bartlett said. �They aren't huge themselves, so they love � the fact that these monsters walked the earth.�

They love trains, love dinosaurs; now they have both.