Physical Material Strength

Hi Folks

I have had pretty good success with making a physics based rail simulation,
however one of the problems I am facing is the strength of physical materials and collision meshes.

To elaborate I have a three link chain coupling, rigid meshes connected by physic constraints.
It works as real life, drop the chain over a hook and can pull it around OK.

Couple extra wagons up and the strain on the links obviously increases as you pull the load.
Until such a point that they cannot take the strain and it separates.

I have played around with physical materials, restitution and the like, also the constraint linear and angular strengths, with limited success.

The one thing that does work is if I increase the mass of the links substantially.
This then leaves to other problems, such as

Increase the mass of a link from 12kg(realistic) to say 150 kg and the links are bullet proof.
However as they swing around their mass then rocks a 15,000kg wagon around… Fully sprung and damped

The mass multiplier does not seem to have any effect

Any ideas on how to get the high mass strength without the weight?

Regards

Paul G