Note for the bell you want a non repeating electro-mechanical type. Easiest place to find these is in the doorbell isle of your local home improvement store. Most have a display where you can test the bells. Look for one that strikes a single note when the button is pressed and held down. One that continues to ring when held down will not work for this project. Also note most bells have connections for two buttons. Usually the "front door" button will strike two different chimes where the "back door" normally strikes a single chime. Other than that pick the bell that sounds best to you. Also if you need or want to you could make a bell from scratch you will need a spring loaded solenoid rated at the nominal voltage of your train transformer (~18V works for most O gauge stuff) and a bell or metal strip to strike.