Trying Out DCC!?

A purchase at H&R Trains in Tampa, FL,  over Thanksgiving was an MRC DCC system.  It was supposed to be simple and "intuitive" to connect.  The only problem was I needed to have easy access to the DC wiring to run Thomas for the newest granddaughter!  So, I hoped to install a switch to disconnect the DC system and then connect the DCC system.  That seemed simple enough until I stopped by Radio Shack to find the appropriate switch.  I finally picked up a blade-type switch; if nothing else, I could see what should have been happening.  I left all of the red (-) wires connected and inserted the switch into the black (+) side like this:

Before

After

Then I went to work on the locomotives.  The Southern F3 A/B set had come with decoders that I had removed and replaced with shunts for DC operation.  I replaced one in the F3 A unit and put one in the SCL loco The Wife gave me as a gift.

SCL Shunt Disconnected

Decoder Ready to Connect

Job Completed

 I connected the DCC system under the control panel to the switch (black, +) and to the gang bus (red -).

When I plugged it in, I got an "ovrld" signal; I assumed that was "overload".  I disconnected the DC red (-) wire from the gang bus and got rid of the overload.  But the locos wouldn't run!  I got out the ole multimeter and started testing from the power supply through the switch to the Atlas controllers.  No current downstream of the controllers!?!  On a whim, I moved the top Atlas direction switch.  Lo and behold, it worked.  Now, should I figure out why or just don't mess with success?