Hello, I am new to this site and loving it. I did a bit of Electronics in 1995 where I made a flashing led circuit with 2 led's and it was cool. My other hobby is model trains and I would like to put 2 small led's in buffer stops and make them flash and run it off a DC train controller. Does anyone have a simple circuit that would do the job for this? I am basically a noob at electronics but I can follow a simple circuit.
Thank you.
Gary.
commanderkelly Welcome to the wonderful world of Arduino. The Uno is my favorite. You'll have a lot of fun integrating it into your railroad. Prepare to lose even more sleep due to your hobby. 
Hello I tryed that circuit you showed me with the push butten, I also added a led from pin 13 to the breadboard with a 100Ohm Res. 1. Off 2. On 
And a scale model signalling hut to keep the Arduino hidden in 
Keep asking if you get stuck.
You should be able to do all that with your Arduino. Currently you are using some pins as outputs to the LEDs, as well as that you can set pins up as inputs.
Mechanical Switch
The simple circuit below shows such a switch wired in [the full article is on the Arduino website: https://www.arduino.cc/en/tutorial/button ].
With the switch open circuit the Arduino reads a low voltage on the pin 2 as this is effectively grounded through the resistor. When the switch is pushed the +5v appears on the input. In your Arduino code you would probably loop around in a 'WHILE LOOP' constantly reading the switch and adjusting the LED state accordingly. That is the fun part as you are effectively crafting the code to do what you want.
A simple switch would need pushing and I guess physical contact between train and switch could easily scratch or derail the train. You could maybe get one under the track that detects the extra weight as the train passes over.
Contactless Switch is Better
Probably a better way is to go for a contactless setup but it would be slightly more difficult to understand. There are several ways including Hall Effect switches but my preferred route would be to use an Infra-Red (IR) diode and photo-detector placed either side of the track. The classic bank-burglar beam break setup just in miniature: The IR LED would run from the Arduino +5v supply via a suitable resistor - this would emit a constant level of IR light across the track and onto the photo-detector. The photo-diode would need a transistor amplifier but effectively become your switch input....I'll try and find/draw the bits you need but there aren't many.
*** Easier still you could also just buy the sensors (search Adafruit IR Beam Break Sensor) as they incorporate the LED resistor and the photo-detector amplifier. You can see an example of them being used here: https://learn.adafruit.com/ir-breakbeam-sensors/arduino
Coding
For your idea you would need two beam break sensors - one on the buffer approach and the other right up close to the buffer. When ready we'll gladly help with any code suggestions for you as well 
