8-pin vs 9-pin decoder harnesses

A question from someone who is not decoder-savvy:
Why do some HO locomotives come with 8-pin DCC harnesses (for example,
the Atlas SD35) versus some that come with a 9-pin harness (for example, the Walthers RS-2)? What's the difference in operation of the plug? How does that limit decoder choices?
Atlas SD35: http://www.atlasrr.com/HOLoco/hosd35c.htm "... NMRA 8-pin plug for DCC ..."
Walthers RS-2: http://www.walthers.com/exec/productinfo/920-35127 "... factory-installed 9-pin DCC harness"
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Mark Mathu schrieb am 31.08.2010 19:54:

The 8-pin is more common in Europe while the 9-pin is widely used in the US. In fact I would have serious problems to get a 9-pin decoder over here in Germany (except an US decoder imported by a special US model railroad store). The 8-pin plug has often only 7 line connected. PA1 and PA2 (green and brown) are often not or only green (PA1) is connected. The 9-pin plus is fine to have all lines and usually has all lines connected.
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On Tue, 31 Aug 2010 20:21:01 +0200, Reinhard Peters

Can a person use an 8-pin decoder in a 9-pin socket? What are the results?
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wrote:

The form factors are different, the 8-pin is 2 rows of 4 pins, while the 9-pin is 1 row of 9 (much smaller) pins. The difference is merely physical, adapters are available to go from one to the other.
Pictures are available here: http://www.litchfieldstation.com/DCC-University/SelectHarness.htm
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Yes, with a 8 to 9 pin adapter harness. In fact I buy 'blank' eight pin male plugs and solder up my own harness. I use a lot of Train Control System T1 decoders which are 9 pin but they came with the female plug on a hardwire harness to be soldered into the model. But I take the eight pin blanks, color match the harness to the plug and solder it up. Voila - 9 pin decoder on an eight pin harness. And of course you can buy those harnesses premade from TCS and others.
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