Best DC wiring: Common- or Two-Rail?

[frank-in-spokane]

I'm preparing to lay track soon on my first MRR in 35+ years, a simple N-scale oval in 5' x 2.5' w/one passing track and 4 sidings. Turnouts are Peco Insul-frog, and I'm going w/DC cab control.

My Q. is, should I use Common-Rail or Two-Rail wiring?

Here are the concerns behind my Q.:

1) MY LARGER CONCERN: DC transformers reverse loco direction by reversing polarity to the rails.

So assuming:

- You wire the common rail to the terminals on both Cab A and Cab B terminals which are (-) when their direction switches are set the same; and

- Cabs A & B are both running the same direction according to their direction control switches (i.e., the (-) terminals of both packs are connected to the common rail) ...

If Cab B's operator throws his direction switch in order to reverse, it seems to me that the common rail is now connected to the (-) terminal of Cab A and the (+) terminal of Cab B.

So why won't there be either a) a short circuit, or b) a "neutralizing" of track voltage across the layout (due to Cabs A & B "working against each other" polarity-wise)?

I know Common-Rail wiring "works," since Atlas has been around for centuries! But why doesn't reversing one cab cause any problems?

2) MY LESSER CONCERN: Kalmbach's book "Easy Model Railroad Wiring" describes "all-live" vs. "power-routing" turnouts as follows:

a) RE. "ALL-LIVE" TURNOUTS: "The combination of insulated frogs and jumpers ... keeps the rails properly insulated and connected through all sorts of turnout arrangements, and the power-feed wires from the power pack ... can be connected anywhere." The accompanying diagram shows how "both routes [are] always live whichever way turnout is thrown."

b) RE. "POWER-ROUTING" TURNOUTS: "Orienting these turnouts frog-to-frog causes short circuits, as do feeder wires attached on [their] frog side ... To use these turnouts you have to insulate rails in certain places with gaps or plastic rail joiners, and be careful where you attach feeders."

I believe the book's description to be misleading (although certainly unintentionally!), because Peco Insul-frogs both a) have an insulated frog *and* b) are power-routing.

So do I perceive correctly that there is no concern for shorts due to frog-to-frog orientation or frog-side attachment of feeder wires?

In summary: My layout is small and simple, so although I'd prefer to go with Common Rail wiring, I'm not worried about the cost or complexity of going Two-Rail if necessary. I'm more concerned that everything go together as (reasonably!) smoothly as possible.

Thanks in advance for your help!

 

[MRLdave]

In my experience the problem you mention will occur if you are using multiple power packs......my layout uses 5 packs, and as you mention, if you have adjacent blocks powered by different packs going different directions you will have problems. In my case, the power to the loco was the combination of the 2 packs added together......in this case one number positive and one number negative.....the locos in both blocks went in the direction of whichever pack had the higher setting, reduced by the setting on the other pack. In other words, if Cab A was set for east at 7, and Cab B was set for west at 4 , both locos would go east at 3. Like you, I know common rail is supposed to work, but it didn't for me.

With power routing turnouts, I know there can be issues also......on our NTRAK layout, we use Pecos, and each line is isolated (no crossovers between lines) but there are sidings on each line, and if even one turnout is thrown wrong it brings the line to a screeching halt....both turnouts on a siding must be thrown or it creates a short

 

[CSX Robert]

...
If Cab B's operator throws his direction switch in order to reverse, it seems to me that the common rail is now connected to the (-) terminal of Cab A and the (+) terminal of Cab B.

So why won't there be either a) a short circuit, or b) a "neutralizing" of track voltage across the layout (due to Cabs A & B "working against each other" polarity-wise)?

I know Common-Rail wiring "works," since Atlas has been around for centuries! But why doesn't reversing one cab cause any problems?
...

Before I explain how it works, I want to point out that it only works if your cabs have separate power supplies. If you have cabs operating off of a common power supply, it will not work. The transformer that steps the 120V line power down to the cab voltage also isolates each cab and is what allows common rail wiring to work.

Now, on to how it works. As an example, let's say you have two cabs set to run trains, cab "A" set to forward and cab "B" set to reverse. Each cab has two terminals, 1 & 2, and you have terminal 2 from each cab attached to the common rail. On cab A terminal 1 is + and terminal 2 is -. While we tend to think of a terminal as being either + or -, if you just look at one terminal by itself, it actually is neither. It only becomes + or - when compared to something else, terminal 1 is + relative to terminal 2 and terminal 2 is - relative to terminal 1, Since cab B is reversed, terminal 1 is - and terminal 2 is +. Since the two cabs are isolated by their transformers, there is no common point of reference between cab A terminal 2 and cab B terminal 2. Since there is no common point of reference, then neither one is + or - relative to the other and you can tie the two together. By tying them together they become a common point. This common point is - relative to cab A terminal 1, but at the same time + relative to cab B terminal 1.

...
2) MY LESSER CONCERN: Kalmbach's book "Easy Model Railroad Wiring" describes "all-live" vs. "power-routing" turnouts as follows:

a) RE. "ALL-LIVE" TURNOUTS: "The combination of insulated frogs and jumpers ... keeps the rails properly insulated and connected through all sorts of turnout arrangements, and the power-feed wires from the power pack ... can be connected anywhere." The accompanying diagram shows how "both routes [are] always live whichever way turnout is thrown."

b) RE. "POWER-ROUTING" TURNOUTS: "Orienting these turnouts frog-to-frog causes short circuits, as do feeder wires attached on [their] frog side ... To use these turnouts you have to insulate rails in certain places with gaps or plastic rail joiners, and be careful where you attach feeders."

I believe the book's description to be misleading (although certainly unintentionally!), because Peco Insul-frogs both a) have an insulated frog *and* b) are power-routing.

So do I perceive correctly that there is no concern for shorts due to frog-to-frog orientation or frog-side attachment of feeder wires?...

I've never used Peco Insulfrog turnouts so I am not sure how they operate, but there are at least two different type of "power routing" turnouts. The more common type has the inside rails beyond the frog(and the frog itself) common to each other and switches both them to the matching rail depending on which way the turnout is thrown. This type requires insulating gaps to prevent short circuits. The other type merely cuts power to one rail or the other(this is how Kato #6 N-scale switches work) and does not cause shorts.