2 RAIL O SCALE OPERATORS...command control

With 2 rail you do have to have something to _correct_ the polarity as a train passes through a reverse loop, even though the decoders in the locomotive does not care about the polarity.  This can be a manual toggle you throw yourself (NOT recommended), or by either the boosters or a separate device to automatically change the polarity when a short is detected.

The same goes if you want to power the frogs on your switches.  This can be done as part of the mechanism you are throwing the switch (e.g. extra contacts on your switch machine) or via a frog juicer (which also looks for a short and changes the polarity of the frog to attempt to remove it).

There are electonic devices sold by a number of manufacturers, for example Digitrax AR1 for 24.95, that automatically reverse the polarity of the track as the loco hits the end of the reversing loop.   They do this by sensing the short and reacting faster than the circuit breakers in the boosters (or breaker boards).   

I have one of the devices PRRJIM is talking about. I have the PSX-AR by DCC Specialties which is designed for use with DCC. I don't have a reverse loop but I use it for my turntable. It works great, the locomotive doesn't even slow down when it crosses the gap and the polarity is incorrect. I paid $50 for it. I was just looking at Tony's website and DCC Specialties makes a PSX-AR-AC which is designed for someone using AC in a 2 rail environment. In other words if you were running conventional AC on 2 rail track you could use this device for turntables, wyes, and reverse loops. It might also work with DCS and TMCC but I am not sure about that. The description only mentions conventional AC. 

Tam Valley's Frog Juicer.  Undetectable engine hesitation when traversing a wye. Absolutely smooth transition.  About $30.   ($15.00 each, If it comes on a green board I always  buy a spare!)

Tom, who makes the Frog Juicer?

With DCS, there's an issue on Proto-2 engines with respect to track polarity -- i.e., the Proto-2 engines can only pick up from the hot side. Proto-3 engines don't have the polarity issue. The work-around for that is a modern version of the old-school toggle switch. Azatrax makes optical sensors that reverse the polarity as the train enters/leaves a reversing loop. The site is http://www.azatrax.com. John is a good guy and his products feature application notes.

I have always wondered about the big deal made on polarity with older PS2 MTH engines. If you want to reverse the way the engine is pointed as you put it on the track,  you throw a switch. Is that, that hard to do?

 If you had a polarity reverser connected, wouldn't that keep the polarity correct for PS2 engines? Isn't that the same for all engines?

Of course I maybe wrong about this,.... again.

Engineer-Joe posted:

I have always wondered about the big deal made on polarity with older PS2 MTH engines. If you want to reverse the way the engine is pointed as you put it on the track,  you throw a switch. Is that, that hard to do?

 If you had a polarity reverser connected, wouldn't that keep the polarity correct for PS2 engines? Isn't that the same for all engines?

Of course I maybe wrong about this,.... again.

Basically, when the train reverses back onto the main, the polarity on the main needs to be switched to prevent a short from the reverse loop. When the train gets into the other reverse loop, the mainline needs to be reset back. Think DC polarity operation here because of the hot-rail requirement. I'm not sure of how the Frog Juicers do it, but I didn't see relays mounted on the boards, so I don't THINK they physically reverse the rails.

Yes, that is what I am thinking Matt. So if the polarity switch is thrown correctly on a PS2 engine and it runs onto a block that reverses automatically, what would be the difference? Wouldn't the hot rail stay on the same side of the engine?

On my American Flyer layout I have a number of the PSX-AR-AC's mentioned above by Phil. They are used on the turntable and reverse loops. Technically in AC there is no polarity, it is more like phasing. The wires powering the rails must be flipped similar to DC layouts when there are reverse loops in the track plan.

The layout is Legacy equipped and the PSX-AR-AC boards are compatible with the Legacy system. In 2 rail AC it is advisable to put the Legacy signal on both rails. This is done with .15uF capacitors. The turnout frogs are powered through relays to assure they are powered correctly depending on the direction the turnout is thrown. The PSX-AR-AC boards feed the track power to the isolated reverse loop blocks, detect the connection orientation required as the train enters the block and then flips the wire connections to the rails as required. No operator intervention required.

THANK YOU ALL for the responses.  Now I know.  :-)

Phil, see above.