Picture of DIY train crossing bell
I had a request from my dad to come up with a train crossing bell. He recently got back into working with Lionel trains and wanted a bell to go along with his crossing guards. 

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Step 1: Required parts

Picture of Required parts
Note all the parts for this project were sourced off my workbench. Most parts can easily be substituted with other values if needed.

1x dual screw terminal block (optional) 
 5x 1N4002 diode
1x 1000uF 35V electrolytic capacitor 
1x 78L05z regulator (to-92 package) 
1x 100uF 16V electrolytic capacitor
1x 22uF 50V electrolytic capacitor
1x 5.1k Ohm resistor 1/4W 
1x 22k Ohm resistor 1/4W
1x NE555 timer IC
1x 8 pin dip socket
1x 100nF 50V ceramic capacitor
1x 120 ohm resistor 1/4W
1x 5mm red LED
1x 1k Ohm resistor 
1x 2N3904 NPN transistor (to-92 package) 
1x 10k Ohm resistor 1/4W
1x IRF640 N channel FET (to-220 package) 
hookup wire

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.  
Connect pad3 to the transformer fixed voltage output and pad4 to the insulated rail in a section of insulated rail Lionel track. Bell will ring when a train car is on the insulated track section.