This is a silly question.

Simple answer - Anywhere
Slightly more drawn out answer - The beauty of Parkour is that with a little immagination, just about any object can be turned into a training obtacle/opportunity. Flat walkways can be used for practicing your broad/precision jump (aim for the line). A small rail has endless possibilities for vaults, balance, even pullups! Train on ANYTHING and EVERYTHING! When you have spent enough time training and practicing, your view of the world actually begins to change, you stop seeing simple rails and start seeing awesome places to practice. All those comments in the "Are you a traceur" forum and the like saying things like "You see a sweet rail and have a ParkourGasm" are quite serious. I quite literally have to be pulled away from places that look fun to train in. Ive spent half an hour on my way home at a tiny picnic bench, practicing all sorts of vaults.

So... Use your mind.

As the above posters said, parkour makes the world your playground; learning to see it like that is part of your parkour journey. Look around you there are opportunities everywhere, everyday when you go somewhere.
I don't see rails, steps or walls anymore...

extra: Thanks for the scrolling signature idea Jason