Many manufacturers of model trains now have built-in cavities where speakers can be located. In some instances you can make a space for a speaker in the fuel tank on a diesel locomotive. Some of the older diesel locomotives might be trickier. Steam engines will typically have a tender (unless they are a tank engine) where you can locate a speaker. I usually install the speakers face down as this generally gives off the best sound. I cover the speakers with a double thickness of panty hose to avoid metal fragments getting attracted to the speaker magnet.