additional info from IRFCA FAQ
Catenaries : IR uses catenaries of the
constant-tension type. At one end of each
section of the catenary the cable connects
to a pulley block or winch system...
more... which
then connects to a cable that goes over a
pulley and is terminated by a hanging
counterweight. The pulley and weight
combination ensures that the catenary
cable maintains the same tension
regardless of the ambient temperature and
the consequent expansion or contraction of
the cable. This avoids problems with the
catenary sagging too much in hot weather,
or, if the tension is too high, snapping in
cold weather. The pulley block or winch
arrangement provides a suitable reduction
ratio between the change in the catenary's
length and the distance the counterweight
moves. A reduction ratio of about 5 is
common.
The tension of the catenary cable in most
cases is kept close to 1000kg at 35°C.
Maximum tension length for the catenary is
2000m, although in practice it is usually
shorter. It is important that the tension be
within certain precise bounds for
mechanical reasons: the moving
pantograph creates a shock wave in the
catenary that travels along the cable; its
speed (the critical velocity) is determined
by the tension in the catenary, and if it is
less than the speed of the pantograph, the
cable will be prone to buckling and
snapping, or else the pantograph loses
contact with the wire as it oscillates in
resonance, and current collection fails. For
this reason the tension also has to be set
so that the critical velocity is higher than
the maximum speed of trains on the
section. The pressure of the pantograph
pan against the contact wire is us
