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I think a lot of that depends upon the quality of the trackwork and the turnouts.

It also depends upon whether or not both the track and the locomotive are within NMRA or NEM standards.

Sometimes it can be caused on curves by coupler swing limits, something hanging up the coupler and not allowing it to return to center upon exiting a curve. I have an ÖBB passenger coach that had a step getting in the way of the coupler arm swing and was not allowing full left and right travel. Simple bending the steps a bit took care of it.

S-curves can cause it too. You want to have at least a car length of straight track between reversing curves. More is better.

Not enough weight can also be a cause.

Lots of things to examine before an accurate diagnosis of a problem can be made to rectify the problem.
 

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It has to do with the drive mechanism and/or flange alignment on the wheels.

Mechanical drives don't work quite the same way in reverse and in forward directions due to construction/assembly variances and even due to machining. Gear tower slop will be present in one direction but be almost absent in the other.

Even when wheel flanges are misaligned somewhat, they will trail reasonably well. So, all wheels make it through a location. Reverse and make the misaligned flange lead and you get a whole 'nuther ball o' wax.

Graphically, consider what I mean:

Aligned-

<==)(=======)(==>

<==)(=======)(==>


Misaligned -


<=)(========)(==>

<==)(========)(=>


Both axles in each case have wheels in gauge, but they obviously aren't aligned on their shafts back to front. They won't track well.
 

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Why do some locos have a tendency to derail going in one direction over the other?
Question is too vague for me.
What particular locomotive are you asking about?
Steam
Steam with leading and/or trailing trucks
Diesel four wheel trucks, six wheel trucks
.
Coupler type on your equipment?
long shank or short
Kadee couplers have a metal 'air hose' hanging down which can catch on thing in one direction, but not the other (if not adjusted properly)
 

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Locos that do this are head scratchers sometimes.

Especially new locos.

Sometime back I bought a new Bachmann FA unit,
identical to the one I owned. Immediately when I
consisted them, the new F would derail here or there
with no consistent reason obvious when it was the
lead unit. However, when I made it the trailing loco
all was well...after running some time now it has lost
it's derailing habit. So my guess, it just takes time
to break in a new loco.

Don
 

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Discussion Starter #9
My "problem" loco is a 4 axle Atlas Alco RS1.
It only does it in the reverse direction (front facing forward).
I have a Mantua articulated 2-6-6-2 and it runs fine. I figured if any should have a problem it would be that one. ???
 

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I read about an issue similar to this on hornby website, but it wasnt a backward/forward issue, it was a left right issue, apparently hornby (and maybe some other manufacturers ) put a small piece of packing material, between front bogie and body, it's very hidden and unless removed, bogie only wants to swing one direction.. not saying this is your issue, but worth a look...
 
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