What is the name of this movie?

Need help trying to figure out what movie it is. Im pretty sure it came out before 2000s, probaly 1980s because of the quality of the special effects. I think there was time travel involved. A teenager travels back to time, ends up in medieval times.Tyrant King has a 'dragon', turns out to be a trex that ended up in that time.The boy finds a green triceratops to help fight against the trex. I remember it being part of a series, i think. Anybody what movie its. The "Time machine" is either a van or train don't really remember. What i do remember is the "time warp" being a tunnel with blue/purple walls like a wormhole. It wasnt a comedy, i mean it did have some comic relief.