“If my podcast is about a subject (like “Trains”), can I play copyrighted music about trains, like Tom Waits’ “Train song” as part of my podcast?”
Short answer: it depends.
Generally, you’ll need permission to use copyrighted works in your project. However, there are exceptions to when you can use a copyrighted work even without permission. It depends on what you’re going to be using the copyrighted work for in your project. If it falls under the use of “fair dealing” then you’ll most likely be okay. Usually if the work is used for education, private study, research, parody or satire then it should be okay.
You can find the whole Copyright Act here: http://laws-lois.justice.gc.ca/eng/acts/c-42/