"Model Railroading is fun!" The old motto written and read many times over! I sarcastically said that to myself over and over and over today as I began "tracing" wires to figure out all of a sudden why my "B" transformer for my main line would no longer make the train power on.
I operate conventionally, even though I have a large layout. I did this subscribing to the KISS (Keep it simple stupid) knowing that others will operate this layout in the future.
My power both positive and negative lead from the "B" transformer to a plug in which leads to a power strip which leads to an Atlas Selector.
The "A" transformer powered the train in the block; there fore I knew the wire to the block was correct. I started with my "meter" testing at the transformer, then at the plug in, then at the power strip and finally to the Selector. Yet it still wouldn't work.
All of a sudden I simply take my screwdriver and touch the wire that is connected to the "B" transformer input of the selector. Wham! The engine started rolling down the track. I then continued and tightened the screw.
Some times the problem is the most simple thing.
"Model Railroading is fun!" Ha!!!!!!!!!!!!!!!!!!!!!!!!!
I think I need to tighten my screws! 
