I had an issue with an MTH train that, on occassion, would not be found by the DCS system. And eventually was never able to be read by the DCS. After much head banging and scratching, I came to find, one of the wires from the switch SHOOK LOOSE. I was able to GERRY RIG it. And now it works fine.
A big thank you to you guys who responded to my issue. As I said, I did check the wiring carefully, but apparently not carefully enough. This kind poster put the idea in my head that I should probably check again, paying particular attention to the dcc/dcs switch wires. Once again, I glanced at them and they seemed fine, but upon close examination, one of the wires had broken off the switch and stiffly stayed in a position where it was difficult to see that it no longer made contact. Bright light and reading glasses allowed me to notice it and once I soldered it back, my ABA came back to life!