christhetrainguy09 posted:...using GRJ module, simply run track power to one terminal and the power drop to the other terminal and plug the rest of the pins up to Arduino and write a little code and bazinga you have a block detection
Well, it depends on your definition of "a little code" and your definition of "bazinga"! 
The IC sensor in GRJ's module uses the relatively recent ACS712 IC chip manufactured by Allego Microsystems which generates a voltage proportional to sensed current. The scale for the 20 Amp sensor chip is 100mV per Amp.
So even if you have a caboose at the end of every consist, let's say its lamp draws 5 Watts. At 18V command voltage, that's less than 300 mA (0.3 Amps). The eBay ACS712 module generates "only" 100mV per Amp...so when the caboose rolls by, the output voltage to the Arduino would be a modest 100mV/Amp x 0.3 Amps = 30 millivolts! I'm not saying 30mV is difficult to detect but to quote GRJ, no job is easier than the one you imagine someone else doing!!! 
As mentioned, the "beauty" of the bridge-rectifier detector method is that even if the current is "only" 0.3 Amps, the output of the detector will be about 1.4 Volts...or some 50 times greater than the eBay sensor! In the OP's case, this 1.4 Volts is used to trigger an opto-isolator (which trips at about 1 Volt).
So, as GRJ points out, if you can't or don't want to use the isolated outer-rail method, you need to consider something like an IR/ITAD detector if occupancy detection is the objective.
