Dan Griffin

Are the axles parallel? Are the wagons weighted? Have you checked the back-to-back?

Yep, yep and yep! parkside tube wagons. heavily weighted, wire loop couplings and Bachmann wheels. i fear they are ebay bound. 

Take a deep breath, and check... You need a mirror . Sit them on the mirror , and see if all 4 feet are firmly on the ground . If they rock - there's your problem . Chassis not square.

You say they are Tubes - which is I think the longest wagon chassis you are likely to encounter. Once you are over a 12' wheelbase the precision required to assemble them exactly square gets really quite demanding (The longer the wheelbase, the more any error in alignment is magnified).

TBH , anything over 12' wheelbase is a strong candidate for building as compensated.

Have you built the chassis "tight"? or is there some slop in the axles, allowing them to float slightly and take up a suitable level? It's the ones that I've built "tight" with no float that have given trouble. You may be able to melt one bearing in a bit with a soldering iron tip to ease one axle out and create a bit of float (Horrible bodge, but can be a life-saver). Alternatively , you may have to remove the axleguards at one end and rebuild with a compensation unit. (Etched W-irons)

I doubt they derail randomly. Find the reason they are doing it, and you'll feel a whole lot better. But building something with this length of wheelbase entirely rigid is sticking out your neck a bit