Charles,
I've also been working on an isolated rail detector using a bridge rectifier and I can validate what Stan2004 and GRJ are advising you. In testing I was using a 'kludged up' bridge rectifier thru an optical isolator and reading an Arduino input pin. With no smoothing capacitor the readings were all over the place. I installed a 330uF / 35V capacitor and the readings became solid digital 0's and 1's (or 0's and 1022/1023 when reading analog). The attached schematic is what I plan to use on my layout. Hope this helps or at least provides some food for thought. Best of luck to ya'.
Barry