If you want a "tinny", "rasping" tone for your model train, then you only need to purchase and fit a low cost speaker from your local consumer electronics store. Typically these come in 27 mm or 57 mm diameters. Use the largest which you can fit in your loco.
For a better quality sound and more volume, the model train speaker needs to be baffled. That means that it needs to be built into the front face of an air tight box so that sound is only heard from the front of the speaker and none from the back. The box can be made of plastic, styrene, wood acrylic, etc. A popular choice is to use the plastic top of a spray paint can.
An easy solution, which ensures high quality sound with lots of deep throbbing bass for your model train speaker, is to purchase an external speaker unit for iPod and MP3 music players and remove the two speaker modules for use in two locos. These are readily available at consumer electronics stores.
4 ohm speakers can also be used but they draw more power and may cause the sound card to shut down intermittently due to overheating. This problem can be avoided by fitting a 2 to 4 ohm resistor in series .
