This project is built to use a minimum of parts to get started quickly. It can easily be expanded in multiple ways if desired.

The matrix keypad from Adafruit comes with some male-male pins allowing the female connector to mate with a female breadboard or circuit board. Place the header onto the keypad connector and plug the pins into the Grand Central pins 1 to 7. The colored face of the keypad should be up as shown in the diagram.

Place a black header male pin to alligator clip from GND on the Grand Central to the amplified speaker TRS connector, the sleeve closest to the plastic of the plug. Take a red jumper, plug into the Grand Central A0 pin and clip the alligator clip to the tip of the speaker plug. This will make a mono channel amplifier connection to one of the speakers of the stereo set.

Ensure the Grand Central is plugged into power, either 9 volts on the barrel connector or maybe connected to the USB port to draw 5 volts (such as a computer connection or a 5V "cell phone recharge" battery.

Power the speakers by placing the speaker USB connector to a suitable 5 volt USB jack. 

Operation

Turn on the power for the speakers if necessary and adjust the volume so sound can be heard when operating.

Press one of the keys on the keypad, from 1 to 8. You should head a sound effect related to trains. Pressing another key will interrupt the playing sound to play the new sound. You can also use the zero (0) key to stop a playing sound.

If the sound cannot be heard, check your speaker volume and connections. Be sure the speakers and Grand Central are powered and that the keypad is plugged in correctly.

Have fun with all the train sounds on your new soundboard!

The attribution for the sounds used are in the file Sound-licenses.txt in the /sounds directory. Two are public domain, the rest are CC3 attribution.

Going Further

You are not limited to these 8 train related sounds. You can take any sound, convert it to a wav file, name it from 01.wav to 08.wav and have it triggered. Use this guide to convert a file to the appropriate format for playing on microcontrollers: mono, 16-bit PCM, 22,050 Hz (or less), WAV format.

You may consider many variations on this project. Some code changes and all 12 keys can play sounds. A bigger keypad or multiple keypads would allow selection of more sound effects. With all the input pins on the Grand Central, a very elaborate soundboard could be created. The limiting factor would then be the amount of space on the Grand Central flash drive for sound files. In that case, the SD card slot could read a large SD-micro card with sounds on it, leading to some truly Grand effects.