Mark Brader wrote:

James Robinson writes:

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.

Er, this is why it *does* make a difference.

If a passenger car weighs P tons, and a locomotive weighs L tons
(where L P), then moving a single locomotive from the front to
the rear increases the total weight behind the Kth vehicle from the
front of the train by L-P tons; and it increases the total weight
behind the Kth passenger car by L tons.

My response was hyperbole, to some extent. I was addressing the
descriptions in the press that focus on the big nasty power car at the
rear of the train, and ignore the fact that the leading carriage had 7
other carriages behind it in addition to the power car.

The power car was not some sort of juggernaut that pushed everything
ahead of it hither and yon, only additional mass that adds to the
momentum behind the leading carriage. To suggest otherwise is to
suggest that trains made up of anything in excess of 8 or 9 carriages
is somehow unsafe.

It is the very essence of what a train is -- a series of vehicles
coupled together. To ascribe the extent of the derailment solely to
the fact that a power car is marshalled at the rear, which some
reports did, demonstrates a misunderstanding of the physics.

Does it mean that the mass from additional carriages are somehow
better than the equivalent mass of a power car? Should trains be
limited in length to a maximum of two carriages, since the additional
mass of one more carraige behind the leading one would cross a
threshold of safety and become unsafe? Would those who advocate the
removal of the trailing power car reverse their views after a tail-end
collision and demand the additional protection of the power car again?

It is one thing to decide that this difference does not pose enough
additional risk to offset the operational benefits; it is quite another
to say that it makes "absolutely no difference" and throw around words
like "complete misunderstanding" while disproving your own point.

The media reports, plus those of many posters to this group
demonstrate a complete misunderstanding of the physics, and ascribe
far more risk to the operation of locomotives at the rear of trains
than is reality.

However, let me rephrase my original statement to reduce the
controversy: Given the many factors involved in collisions and
derailments, the effect of placing a power car at the rear of the
train on the severity of the resulting accident, in comparison to
other factors, is so small as to be inconsequential, or presents no
greater risk than other generally accepted operating practices. Is
that run-on sentence mushy enough?