It sounds like you checked

It sounds like you checked out the loco pretty well, how's the track?  Twists, dips, bumps, improper gauge and dead bugs are leading track causes of derailments.  How tight are the curves?  An SD40 is probably more comfortable on at least 22"radius curves.  Do other 6 axle locos operate over the same track without problem?

I don't have one of the SD40s in operation but once I had an SD40-2 that would not stay on the track.  It turned out the metal tab on top of the truck, the one contacting the frame, was not parallel with the axles, it was slightly bent and causing the truck on the opposite end to derail.  That truck was perfect so I was happy to find and fix that problem.

There is a reason, good luck

Paul

I think Paul's on to something

Chuck:

I think Paul's onto something. I suspect one of your truck towers has a problem. In this case, watch to see if it's always the same truck that derails or if it's both ends. If it's always the same truck then a subtle truck tower flaw could be the issue.

The next step is to swap the truck towers in the loco end-for-end and see if that makes any difference. If not, then my rule is if tweaking doesn't seem to fix things, the next step is to replace it.

The nice thing about Athearn locos is Protopower West carries replacement parts for a very reasonable price. You can get a powertruck tower for the SD40 from PPW for $25 each. If just one end seems to be the problem, then you can get away with replacing just that truck tower. Otherwise if both ends are giving you trouble, then you'll need to replace them both. If both trucks give you problems, I'd also recommend ordering the frame ($11) since it could be twisted and contributing to the problem.

Replacing the frame and both power trucks will run you about $60 - but if the motor and shell are worth saving, then this is one way to probably solve the problem without buying a whole new loco.

Where does it derail,

Where does it derail, switches, curves, or straight track?  Are you running it solo or part of a consist?

It'll have to wait

I think i'll try to swap the trucks end for end and if that doesn't work then I'll have to replace the trucks and the frame. I have 29 locomotives and besides an old athearn AC 4400, it's the only one that gives me trouble. My track radius is mostly between 30" and 36" with one section 24" and I've layed the track were as it is level from side to side except for one section of super elevated rail, which is intentionally built that way.  All the other  six axle locomotives handle the track well except for theAC 4400 and the  SD-40. Sure is a sad day when a $99 locomotve costs an extra $60.  Athearn has been known for sending out a lemon and i usually get them once in awhile.

I'll have to park it in the yard until i can get to it. Got an undecorated Bachman Spectrum SD-45 on lay away at the hobby shop that i'll pick up sometime next week (Got a lot of SP bloody nose decals, SP scarlett, and Dark Lark gray paint I need to use) and an Atlas GE 28B-7 in the SP paint scheme DCC w/ sound locomotive coming in the mail soon.

Strip the locomotive down to

Fixed at last!

          Started tinkering with the SD40-2 (yes, it's a SD40-2 forgot the -2 part) tonight and it's finally running down the rails without any derailments.  I don't know what i did in the procedure but i'm guessing there was a wheel out of alignment between the plastic gear and the metal axle which was making it jump the track in curves.

            I pulled both trucks and swapped front to back, re-checked the wheel gauge, and checked the frame but it was straight as could be.  I did a bit more light grinding on the frame where the bolster meets the frame and in a couple of tight places between the fuel tank and the sideframe that seemed to be touching together when i swiveled the truck back and forth but it still was derailing. I finally started swapping out each wheel set with an old one in my collection of Athearn junk locomotive parts and when i swapped out  the rear wheel set on the front truck, it ran like clockwork. (And my wife calls me a pack rat. Maybe that's why I keep what others consider junk. ) 

         Sometimes these things are tricky to figure out but then again, that's the fun and sometimes challenging part of this hobby. thanks for all your input guys. ....chuck

No worries Chuck, I have part

I'd thow out those SD40 wheel sets ...

Chuck:

You might want to throw out those bad SD40 wheelsets, though. Since you know they gave you trouble, unless you can tell what's wrong with them, it's best to just ditch them. You don't want them to end up back on some other loco in the future and start causing problems again.

Good to hear you solved the issue. As you have discovered, when something's acting up around wheels and track, if you can't seem to solve the issue then it's best to just replace it. If the equipment is tracking poorly and your checks don't seem to reveal any obvious problems - then replace the wheelsets. If the problem continues, move up a level and replace the trucks too - and so on. Sometimes you even have to remove the equipment from service if you haven't found the issue once you replace the trucks and frame/underbody and the issue remains.

Similar thing with track/turnouts. If you check the gauge and alignment and all looks fine - then I just remove the track/turnout and re-lay it with a new replacement. Most of the time that fixes the problem.

When I do this, I can't always say why we had an issue because everything looked okay with what I could measure. No doubt it's something in the relationship of parts that isn't easy to measure that's the issue - and when you replace the part you create a brand new set of relationships and that's often the solution when you can't find it any other way.

most common problem with the sd40 series and sd40-2 series

with the sd38 all the way up to the sd45t-2 is that the wiring to the truck is tight but not enough to really notice it just looking at it. I have had this problem with most of my sd40 and sd40-2 series units not counting the guys at the train club I belong to all I really did was to readjust the wiring to the trucks and presto units fixed. Most of the ones I had fixed the trucks were pulled just enough that going into a turn or coming out the would jump and like I said this is the most common problem I have encountered