II - History of Ericsson - History of Ericsson

selector lines of their own for particular telephone requirements. This desire
is easy to understand and can be traced to the days before automatic operation
was introduced. The gradual additions of telephone lines along the track
resulted in those days often to special uses for each line. It has, however,
proved advisable to retain certain special lines even after the introduction of
automatic operation and after the heavy increase in the use of the railway
service telephony. A very typical instance is the track telephone line; another
is the power telephone when the track is electrified. These two lines fully
retain their character of specially equipped individual line pairs.
Selector lines for the special telephone requirements of
the track
Track telephone
The track telephone line, which incidentally this year (1955) has been in
existence in Swedish railway engineering for just half a century, is the special
service line for the track maintenance. The communications on the track
telephone, thus, refer to orders and information between the section master
and the track personnel regarding maintenance and other activities along the
track, information between stations and the same personnel regarding alterations
of the time table, special trains and similar matters and to a certain extent
also calls of more local character. Nowadays the track line is constructed as
a selective calling system. The line is sectionalized comparatively easily which
is due on the one hand to the extensive number of telephone positions and
on the other to the sectioning mostly following the division of the track
maintenance. It should be connected to a great number of service posts other
than the station offices and the linemen's huts. There are also the section
masters' offices, stores and similar places.
Power telephone
With reference to the other of the above mentioned special lines—the
power telephone—this is, as the name implies, the special service line for
the electric traction system. In character it is very similar to the track telephone
as regards the large number of instruments connected and comparatively
close sectioning, but it is nevertheless difficult to equip it quite in the same way
as the track telephone. Naturally the power telephone should by priority be
available to the power distribution centres (the converter stations) and should
in fact be dominated by them. This line communicates information regarding
faults in the power distribution network and orders referring to fault segregation,
cuts and circuit rearrangements. It is, therefore, of utmost importance
to the traffic operations that the power telephone line is always ready and
available for its special purpose.
From the above follows that the power telephone line if possible should
be ssctionalized at the power feed boundaries for the track electricity supply.
A selective calling system has the advantage that cooperation between different
sections can be easily arranged by means of a code digit and independent
of manual switching. Although normally a suitable sectioning means that the
bulk of the calls is restricted within the section itself (including the principal
station), such cooperation with adjacent sections is nevertheless of great
importance.
As the two special lines mentioned above are only intended for local traffic
and to some extent to calls of short distance character, the transmission
properties of the lines are not of great importance. It is possible to use
comparatively thin (but loaded) line pairs. Amplifiers can be dispensed with.
Consideration has, however, to be paid to the common call which will occur
on these lines and which is intended for special instructions and information.
As this means that a great number of instruments on the section will be
simultaneously connected to the line, the impedance of the selector instrument
must be matched for such a common call. L M Ericsson has developed a
special instrument for this purpose.