RayL posted:... I can see that you and I are going to be conversing on these and other technical matters over time.
Let the games begin! 
As I said, I've posted extensively on the topic of IR occupancy sensing though mostly from a DIY, roll-your-own approach. I am still very much interested in peeking inside the MTH ITAD to better answer recurring questions on how it should be hooked up to [fill-in-the-blank].
If you are going the Arduino approach with discrete IR LEDs and IR phototransistors, this can dramatically drop out-of-pocket cost vs. using ITADs or modules from Azatrax and the like.
As shown above, an LED-phototransistor pair is about 25 cents. Each would need a resistor to connect to a 5V Arduino input/output pin. Note in the 2nd example eBay has a board module with the resistors on it for a bit more. Additional modules shown have the transmitter-receiver pair with additional electronics with features such as a trimpot to adjust the LED current (much like the Range adjustment on an ITAD).
As you have witnessed first-hand, there can be issues with ambient Infrared energy masquerading as a reflection from a passing train. More sophisticated ITAD devices (such as the Lionel 153IR) pulse the IR LED at some frequency (in the kHz or thousands of times per sec)...and then have the receiver "tuned" to look for IR reflections at this magic frequency. Since ambient light would not be pulsing at this magic frequency, ITADs using this method generally perform better. There are even eBay modules that incorporate these so-called modulated IR LEDs and matching detectors.
Above is from an earlier thread so prices/availability are questionable but at least give you an idea of what's out there. If you are a hard-core programmer, you can write code to do the LED pulsing (on the transmit side) and frequency detection (on the receive side); this allows you to use the naked components shown earlier. What the above modules do is simply let you send a simple "ON" command to start the modulation, and returns a simple "ON/OFF" digital signal indicating detection of the modulated reflection.
