You should consult with a trademark lawyer (I'm not a lawyer), but my understanding from lawyers who've trademarked for me is that you must have used (or will use via the intention to use filing) the mark in commerce.

If you have not, then the mark is not active. Because Transistor fm has used their mark in commerce, then they will hold rights to it already even if they have not officially trademarked (not sure if this is so). So a useful way to handle this is to quickly make your first sale if you're trademarking something new (and issue an invoice).

They can also contest your mark during the trademarking process.

The above is the way it works in the US; not sure about other countries.

Also if you are able to successfully trademark Transistor FM and then sue transistor.fm, the suit is unlikely to go anywhere since transistor.fm can easily show they have been using that domain before your trademark. And you may also lose it. Disclaimer IANAL.

You have to show you are using it in commerce and there is a period of time where you can dispute a trademark once its filed. The trademark officer also manually reviews the trademark to verify its not too common (think common words like paper or keyboard) and what is currently in use.

*Not a lawyer, only filled a trademark application myself.