50x19x15mm (without the lugs which can be cut off to save space) 8ohm speaker. It offers more bass than regular bass reflex speakers. It also works with Loksound 4 and Zimo decoders. 

I have fitted it successfully to all types of Hornby TTS diesel apart from the 08. In the 20 it fits perfectly if you remove the existing speaker enclosure. In the 37, 40 and 31 it fits easily in the gap between the glazing. It would also be useful in many other models.

Please see my other items for more speakers. I also offer a fitting service for anyone who wants help fitting speakers.