If it’s a Lionel 5906 button follow the easy instructions, use the clip on wires under the track and a wire nut to connect the 5906 to it. Your 154 flasher and track clip will not connect to fastrack you could add a regular section of track for the clip or check with your local lionel dealer to see what he has that will make it work.
That helps very much… It did not come with a set of instructions. I think my hobby store might buy some things in bulk and placed them in there own bags. I’ll try that when I get home.
http://www.lionel.com/media/servicedocuments/6951_5906soundbutton.pdf
Here are the instructions from the Lionel website.