Tim Christian wrote:

Braking and power control not withstanding, a heavy weight at the rear of a
train is not good news when it has to stop in a hurry,

It makes absolutely no difference what the distribution of weight in the
train is when stopping in a hurry. The suggestion that the locomotive in
the rear is somehow a problem demonstrates a complete misunderstanding
of the physics involved.

The issue is the total mass of the train behind a derailed vehicle,
which includes the mass of the coaches as well as the power car. That
total mass is what creates the tendency to jackknife. The only way to
avoid it is to run separate, individual vehicles, since there would then
be nothing to push from behind. Individual vehicles are what run on
highways. Trains run on tracks.

but a heavy weight at the front means a better chance of staying
upright and, potentially, more protection for the guy at the sharp end.

That is true, since a heavy vehicle is more likely to remain on the
rails, rather than be lifted up in a collision and derail. However, just
because a vehicle is heavy doesn't necessarily mean that it offers more
protection. I acknowledge that you said "potentially", since the weight
can be from other things than extra strength applied to the front
structure of the vehicle, which would provide the necessary protection.