I have two P2K SD9 six axle diesels. There’s one place on my layout, coming off a turnout, where one of the SD9s always derails. The other never derails there, nor do about 20 other engines that traverse it with no problem.
I compared the two SD9s to try to figure out the difference. All wheels are in gauge. The trucks flex in every direction exactly the same. I tore apart the derailing one to make sure there wasn’t a pinched or loose wire that might be catching when the truck pivots.
As far as I can tell, the engines are identical in every way, yet one derails every time, while the other sails right through every time. It looks like the “bad” diesel is rocking back on axles 2 and 3, allowing 1 to climb the rail in that spot, yet the other one does not do that.
Any ideas on something to try or something I missed?
Brand, type and frog number of the turnout? Atlas sometimes cast their plsstic gusrd rails with inadequate flange clearance. Sometimes the frog is not that nor level with the metal rails. Run your NMRA track gauge through the turnout routes.
There’s also thread on here somewhere about a similar problem that did turn out to be one of the power trucks.
It can happen on a truck that the wheels are ‘in gauge’, i.e. the right distance apart, but are slightly off-center one way or the other. Just a thought…
Especially with a split axle type of gearing, where the wheels each have a stub axle that presses in to the center gear. Having the overall wheel gauge correct is important, but so it having the given axle set centered on the gear. Even with correct wheel gauge (and I’m assuming you checked this with an NMRA gauge, not jhust eyeballed it), if one wheelset is off center, it can pull the truck to one side, which leads to it wanting to climb over the points or the frog instead of tracking straight.
One way to help pinpoint this - does it do the same thing in both directions? If you pick the loco up and spin it front to back and run through the same location, does it derail the same way? Or does now the lead truck go through fine but the trailing truck derails?
ANother thing to check is the flatness make sure neither truck is bent or twisted - it doesn;t take much. You can set the loco on a truly flat durface and see if a thin sheet of paper can clip under any of the wheels - a slight twist could keep one wheel slightly raised, again allowing it to pick the points or the frog point and derail.
This is a condition that does not get mentioned often enough. Six wheel diesel locomotive trucks are very sensitive to proper alignment of all three axles. Much more than two axle trucks.
Yep, that would be my thread. It would be worth a read through by the OP. Two identical PA six-axle locos, one runs flawlessly, the other would constantly derail. Turned out, it was the power feed from the truck to the motor. It was pinched and it was holding the truck up off the track where a modest dip occurred.
I had a LL Proto loco acting up, where I had adjusted the wheel set gage, perhaps having replaced axle gears. I found that I had not been precise enough. While the flanges had each been in the NMRA gage slots, I found that attention to both flanges being more precisely centered in the slots mattered. It did not take being off by much to create a problem in that case.
Thanks for all the feedback. The long post by the other member provided some additional ideas for me to try.
I always check wheel gauge first (with NMRA gauge), and then look at the gears, then the wiring inside. Most of my engines are 4-axles, so the 6s don’t get a lot of run time. But when one ran fine and its twin did not, that started my obsession to fix it.
I need to try the “flat surface” parchment paper test on that wheel set next.
I did the paper test and everything appeared fine (other than I realized the clip holding on the worm gear was not snapped all the way down from my reassembly).
Ran the unit without the shell several times to see if I could pinpoint anything. Noticed it was derailing in a slightly different spot than I thought – and also a spot where I had some track leveling issues last year. It’s the oldest part of the layout and I think there may be a bit of sag going on (happens to all of us, right?) I had fixed a low spot on one side where some rolling stock had mysteriously started to derail. This was about 8 ties from that same spot. Got out the mini level and checked – sure enough, the one side was way low. Temporarily shimmed it up with a small screwdriver to see if that would make a difference.
Bingo! Problem SD9 runs through it at nearly top speed now with no issues. I’m guessing that the tolerances on that engine compared to the “good” one were off just enough to allow that front wheel to climb the low spot on the rail. With the low spot gone, it handles it just fine.
it depends on how tight the radius of your curves are and also if your track has any bumps or bent rails. I have had the same issue in the past. heres what you do, find the spot were it is derailing try to swap out a new piece of track for the two spots connecting if that wont fix it you might have to buy curves with a bigger radius.
