Thanks for the info, Hunt.
The plan is to replace the decoder with a TCS 21 pin WOW, hence the question about the dimensions (will the WOW fit it the place allocated to the Bachmann 44957, and the current draw (Bachmann doesn't state the loco current draw, but if the decoder is 1amp capacity, then the loco will only draw up to 1 amp).

All hat said, I have since identified (as you stated) that this is a sound module only and the loco comes with a motion decoder as well, that this module 'piggybacks'.

So I've decided to wait until the loco arrives and see what space is available and where and purchase the WOW after comparing - I'll probably select a a WOW I can hard wire.

Thanks for your responses.
Much appreciated
Dave

Does anyone know the dimensions and current capacity of the 44957 16-bit SoundTraxx® Tsunami® 21 pin sound decoder for the Bachmann On30 2-6-0.
Here is the URL to that decoder.
http://shop.bachmanntrains.com/index.php?main_page=product_info&products_id=6461
cheers
Dave