ZW issue

Are you absolutely sure your transformers are still in phase? What you are attempting to do is very possible - I have more than two ZW transformers phased and have trains cross between them all the time (a the same voltage levels).

I would take a meter and check the actual AC voltage difference between the loops measured from the track itself.  I know you indicated that the 2 transformers are "in phase".  The best way to confirm that is set both loops to the same voltage with the handles and then use a meter to check what you're actually getting on the track.   If phasing is correct, you should get a very small voltage differential between the center rails on either side of the insulated pins/or gaps in the center rails. 

The other possibility is that the pick up rollers are contacting more than the center rails going through the switches.  I have a few Ross switches that for whatever reason, I get pitting marks on the hot center rails between the switch frog and the points.   Not every #6 Ross switch I have installed does this, so I have to assume I have some unique issue on those switches. 

Hmm, notice you said it was phased.  If the engine doesn't go over the crossover, but runs on either loop or the other, that works okay? I was thinking that maybe you didn't connect the ground terminals of the two transformers (the U common post), so ground was going back from the tracks only to 1 transformer, but if that were the case one loop would work and not the other.

Another question, on the switches are the outer rails tied together? It could be you are running into a bad ground, though I can't see why that would cause arcing.

Also, have you tried running different engines and see if you see the same thing? Someone else's post jogged my mind, it could have something to do with the pickup rollers on a specific engine. I would try other engines and see if you see the same thing.

The other thing is, is this the traditional PW ZW or one of the more modern ones? That will help others diagnose the issue.

Thanks gents for the great replies:

My aggregate responses to your questions:

The ZWs are post war 275 W

They are grounded together and yes i checked phasing..by touching a hot wire to the to voltage outputs on the corresponding hot terminals and there is no arching

Voltage on either side of the tracks are the same when set at 10V

The engine runs fine on either side of the track as well as when both loops are powered by one transformer

I have not tried another engine yet

I am not clear on grounds..when it comes to gargraves track, is there something specific that must be done to ensure correct grounding?

Not sure what you meant  are "the switches are the outer rails tied together"  WHAT DOES THIS MEAN EXACTLY IN TERMS OF WIRING?

THE CROSSING OVER SWITCHES ARE WIRED SO THEY BOTH OPERATE TOGETHER.  I HAVE NOT MADE ANY OTHER GROUNDING CONNECTION OTHER THAN TYING THE SWITCH CONTROLs DZ1000s TO THE SWITCH MOTOR AND THEN COMMON TO TRANSFORMER

THANKS

It is sometimes useful w track w wooden ties to put a jumper between the outer rails, which basically gives you ground on both sides ( on old tinplate track the metal ties did this). On a ross switch it is the same, hence my question. It is unlikely this is your problem though. Definitely try another engine, this could be a problem w a specific engine and track geometry, though that doesn't explain why it worked w a single transformer, if an engine part was causing a short on the switch should happen that way too.

I would double check the phasing as someone else said. Given what is in a pw zw, I can't think of what could be incompatible.

While we're brain storming.   The suggestion to try a different locomotive is a good one.  Also, I would thoroughly clean your pick up rollers on the engine that is arcing, assuming that the arcing is occurring on the center rail.   I have seen pick up rollers put on a small firework show which when cleaned with a really good solvent cleaner goes away. 

While touching wires between terminals on two different ZW's is an ok method of determining "in phase" the use of an electrical meter is really better.   

When you say the 2 transformers are "grounded together", I am assuming you mean you have run a good straight wire connection between the U posts on the two transformers ?   I have seen where the posts on ZW's aren't well attached inside the case.  It's been years since I had one opened up, but a way to eliminate that as an issue is to wire all the U Posts together on the outside.  that way even if one of them is loose or making intermittent connection, hard wiring them together negates the need to go look inside.