DC polarity

[Fish]

I'm running into a mental block concerning DC polarity.  I am setting up for 2 cab operation, and for purposes of understanding my question, let's say I have 2 basic ovals connected by turnouts. I have placed plastic rail joiners to create separate blocks, connected everything to an Atlas selector etc; where I am getting hung up is while I am using separate cabs to control Train A on the outer oval and Train B on the inner oval mustn't they always run in the same direction due to the polarity of the control and common tracks?  In other words when you change direction of Train A aren't you changing the polarity of the common rail, so that the polarity of the common rail would conflict with the polarity of the common rail controlling Train B?  Am I making this too complicated for myself?  My hobby shop guy says the polarity only changes on the control rail.  Diagrams in the Atlas wiring manual show positive and negative rails. Some track plans (Atlas) in discussing turnouts specifically talk about "opposing trains."  Any clarification is appreciated. 
PS there are no wyes or reversing loops in my layout.

[Len]

Like the Atlas books show, if you use 'Common Rail' wiring on your layout you only worry about the polarity of the block your power pack is currently controlling. Since you have no wyes or revere loops, the only time there would be a potential conflict is if you connected both power packs to the same block at the same time. If you've wired your block controls correctly, that should never happen.

Len

[Fish]

Thanks Len.  So train direction controlled by separate cabs in each block can be similar or opposite.  Seems the Atlas selector keeps things right.

[Len]

Thanks Len.  So train direction controlled by separate cabs in each block can be similar or opposite.  Seems the Atlas selector keeps things right.

Yes. With the 'Selector' you either connect "Cab A", "Cab B", or no cab to a block. No way to connect both cabs to the same block at the same time if you follow the directions.

Len

[jbrock27]

Fishy you can also accomplish the same thing using Single Pole Double Throw On/Off/On toggle switches.  Some like the look better than the Atlas hardware.  1 switch will control Cab A and Cab B on a block.

[Fish]

Thanks for the reply.

[jbrock27]

You're welcome  .