DCC double crossover with reversing units

[asherspapa]

What is the correct configuration of your reversing units with your DCC double crossover?

[Doneldon]

papa-

By "reversing units" I assume you mean the devices which change track polarity to prevent short circuits when a track reverses on itself. If you mean the units which run a trolley back and forth on the same track these comments do not apply. However, you should be advised that those reversing units will not operate through a crossover or any turnout without some special wiring.

The use of a crossover does not, in and of itself, require a reversing circuit. You only need that if you have a reverse loop, such as a balloon return loop at the end of a right-of-way. However, there are some configurations which use a crossover which do result in a reverse loop which would require the reversing unit. The most frequent reasons for reverse loops are the balloon at the end of a (usually) main line which turns a train around, a wye configuration (may or may not look like a typical wye) and turntables. Most turntables, however, are designed to prevent a reverse polarity situation; check your turntable's documentation.

You can easily determine whether this is true for your proposed track plan by imagining a train traveling over any section of track over which it can also travel in the opposite direction after passing through one or more turnouts. You can also draw your track plan with both rails and see if there is any point at which one rail would lead into the other. It's helpful when doing this to first draw a single line and then draw in the second rail with a different color pen or pencil (or crayon, depending on how advanced you are). If the two colors ever line up together you have a reverse loop.
                                 -- D

[Joe Satnik]

Can you draw the configuration you want using the keyboard?

Example:

_
/  \
l   l
l   l
\_/ oval

l   l
l  /l
l/  l
l   l rt xover

[asherspapa]

The double crossover is intended to allow switching to/from either of two parallel tracks.  Here's the best I can do for a diagram:

    |      |
   /|      |
/  |      |
|   |\   /|
    |  X  |
    |/   \|
    |      |
    |      |

[Doneldon]

Papa-

There's nothing about what you have drawn which requires a reversing unit.

                                                                                                       -- D

[RAM]

I can see where you could need a reversing unit if your track went from A to A  and B to B.  I think you would need two of them. 
   A      A
    |      |
   /|      |
/  |      |
|   |\   /|
    |  X  |
    |/   \|
    |      |
    |      |
   B      B

[Bob_B]

To me that looks like a combination of a left and right crossover and a 30 degree crossing and therefore no reversing required.

[asherspapa]

I guess I should have mentioned that B loops around to B, and A to A in RAM's drawing. And yes, I am referring to Bachman's reversing units, not reversing tracks. 

[jward]

n that case, you'd need to isolate sections a-a and b-b by insulating both rails at each end. you can either use an autoreverser in both sections a-a and b-b, or make the entire sections of track between a-a and b-b your reversing section. the latter requires only one autoreverser instead of two, but remember your longest train must fit entirely within the reversing section.

[asherspapa]

Thanks!

[Doneldon]

Papa-

Aha! Then you do need a reverse unit because it is possible to run trains in both directions on a given stretch of track. You can get along with a single reverse unit because it is impossible for more than one train to use the crossover at a time. If you had two single crossovers instead of the one double crossover, it would be possible, if unlikely, that both would need a reverse correction simultaneously. I'd probably use just one reverse unit for two single crossovers simply because it is so unlikely that it would need to perform on two track sections at exactly the same time. In this case, one will work just fine because you cannot have two trains occupying the double crossover at the same time. Thanks for the clarification.
                                    -- D