(This is not a recommendation for or against Bachmann products!)
1) Starting with the simple DC layout, no problems.
2) Adding turnouts to simple DCC layout, random derailing over turnouts.
3) Fixed one turnout that was causing most of the problems, derailing over turnouts continued.
4) Tried web, found information that the tips (of the direction changing part of these turnouts) needed to be filed to a sharper edge, no noticeable improvements in derailing.
a) Lot of work and big loss of confidence in what was coming from web searches.
5) Investigated problem close-up, no culprits identified.
6) Decided to just live with it, and keep trains short.
7) Much later, became aware of (don’t remember how) the knuckle coupler pin clearance.
a) Cars that derail got inspected, and ones with suspicious pins were bent up so that they could not cause any more derailing.
b) Derailing over turnout’s problem cleared up!
8) Decided that longer trains would take precedence over magnetic uncoupling.
9) Added long trains, random uncoupling.
10) Eventually realized need for quality couplers of the same type on all cars in a long train.
a) Uncoupling problems reduced to acceptable level.
11) Decided to just purchase the higher quality cars, uncoupling problems returned.
12) Decided that these cars all needed to be check right out of the box, and adjusted if needed.
a) Bought and used a coupler gage.
b) Uncoupling problems reduced to acceptable level.
13) Remaining uncoupling problems occurring over just one turnout of the layout “must be the turnout”.
a) Rechecked pin clearance on offending cars, all pasted.
b) Decided to just live with the problem, rather than replace this expense crossover switch.
c) Sometime later realize what was different about this switch. It was at the bottom of a long downhill grade. I remembered from replacing couplers, that at least some of the cars had a good bit of up/down freedom of movement in the couplers.
d) Assume downhill pressure on couplers with too much up/down play is the culprit.
e) Purchase some special washers, and plan on checking and if needed adjust each car.