How do you open the door near the the tracks?

This is an "exit only" door. Between the waterfall and where the train is parked is a big hole with little puffs of smoke coming out. Use a FAST skylander, or one that can fly, go down to the beach to the clamtron. Grab a bomb, zip up the train tracks and drop the bomb in the hole. This is HARD to do. Took me many many tries. Once done, the smoke will stop coming out of the hole. At that point you CAN get a skylander into the hole and drop down into a cavern below. There is a blue butterfly to collect (2% discount) and assorted loot. When you find the exit to leave, you will come out the door near the train track, but I have not found a way to get IN through that door.