Installig railSounds to a hogwarts express

The RailSounds Commander can be installed stand-alone and accomplish what you seek to do.  You will have to add a reed switch and magnet to an axle on the tender and simply follow the installation instructions for conventional operation.  Check on The Electric Railroad for a copy of the installation manual, look on the Manuals link on the main page.

Steve,   There are conventional sounds and Command Controlled sounds.  If the HW stays conventional, you can look at Lionel parts for a Conventional RS or Train Sounds board to add in place of the whistle.  The problem is later versions don't use a battery, so every time you change direction the sounds will start over.

If you can find an early version used from a starter set, they had a conventional railsound set up with battery back up that would work for you.

Or you go the RS CDR setup John mentioned.  G

And the RS Commander does have a battery, and the new ones have great sound.

Wow, thanks for the quick response. Gunnrinnerjon, Do you thing the Railsound commander will fit in the tender. Ive read that its a small car. GGG, can you suggest a place to look for conventional RS board? I see that Lionel has their RailSounds Commander for $100. Will this give me the chug-chugga and bell? Yes, Im really new

After you take the air whistle out of the tender, there's plenty of room for the RS Commander and speaker in the tender.  Here's the picture from the Lionel parts site, note that you'll be removing all the air whistle components. 

Yes, you get chuffing and the whistle and bell.  I don't know if you can trigger the crewtalk from conventional mode, I'd have to look into that.