Reverse loops

[Anonymous]

In designing a layout, should I avoid reverse loops altogether or are there just specific instances to avoid? For instance, suppose you have a long, single trak mainline run with a reverse loop on both ends. How do you run trains in both directions on the common section of the single track with a passing siding? How does the loco "change polarity for the return trip?

[rogerhensley]

The loco doesn't "change polarity for the return trip". You change the polarity of the main. With any reversed loop with normal DC operation, the loop and main are set for the same direction as the loco is going in. While there, you change the main to the opposite direction before the loco comes out. This requires a reversing toggle switch for the main, the siding and each loop.

And, no, Reverse Loope are not something to be avoided. They are just another piece of trackwork that is very useful.

For more information on a number of basics, you might want to take a look at the NMRA Beginner's Pages at: http://www.nmra.org/beginner/

Roger

[Anonymous]

I guess I didn't ask the question very well. I understand that the track has to change polarity. I guess I thought that with DCC there was some automatic way to switch the engine so as to avoid having to use blocks with reverse loops in DCC.

Thanks for the reply

[rogerhensley]

No. You must always change something even with DCC. However, there is a device that will take care of that for you in DCC.

From a reply to a similar question by Stan Ames who has been in on DCC since the NMRA started work on it...
>>The direction of a DC locomotive is relative to the polarity of the track. The direction of a DCC locomotive is absolute and controlled by the decoder and not the polarity of the track. A short is a short is a short. So just like in DC, a reversing loop will have a short either on entry or exit and thus the polarity must be adjusted. This can be accomplished by reversing the main line polarity (common in DC operations) or the reverse loop (since DCC locomotives do not care about the polarity. You can use traditional relays controlled by the turnout, use a DPDT or use a DCC reversing module (sometimes included in a power station/booster) to automatically reverse the polarity so that the track polarity is correct on entry and exit of the reverse loop.

The thing you are looking for is the DCC Reversing Module.

Roger

[Anonymous]

Yes DPDT on the reversing loop is all you need just remember to throw the swithe.