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All the apparatus necessary for the operation and utilization of trunk circuits
is designated by a common word, trunk equipment. It consists of terminal
exchange equipment and, with long trunk circuits, also of repeater equipment,
while with four-wire operation there are also terminal repeaters and voice
frequency equipment. The nature of the trunk equipment is determined to
a high degree by the question whether the connections go over bare wires
on poles along the track or whether they are carried in cables embedded in
the track embankment. The length of trunk circuits is generally so great
that repeated amplification is necessary to bring down the line attenuation
to an admissible figure, i to 2 neper. With aerial circuits of bare wire the
repeater distances are relatively great, 300 to 400 km, while with cable circuits
they stand at 100 to 200 km or less.
Aerial circuits can only be permitted with non-electrified tracks and in exceptional
cases on tracks with electric operation, in which case they must be
moved off from the track to an adequate distance to avoid the noises due to
induction from the trolley lines. For electrified tracks, therefore, cables may
be regarded as normal.
Cable circuits present the advantage over aerial circuits that they are
considerably less exposed to damage and thus cause appreciably less interruption
in working of the telephone communications. Both kinds of circuit
allow of increasing the number of circuits by means of carrier systems. It is
seldom or never a question of multi-channel systems with railway telephone
networks, since two places are seldom linked together by a large number of
trunk circuits even if these run parallel for long stretches. Mostly a singlechannel
system for cables is employed, utilizing the normal repeaters along
the lines. Aerial circuits on the other hand are often equipped with multichannel
systems, chiefly three-channel systems.
Repeaters
To compensate for line attenuation on long circuits repeaters are inserted
at regular intervals along the lines. For each bunch of lines these are assembled
at fixed repeater stations. A distinction is made between terminal repeaters
and intermediate repeaters, as also between through repeaters and cord circuit
repeaters. The through repeaters are connected to definite lines and are thus
comprised in the permanent trunk circuits always available for the establishment
of calls between two places. The cord circuit repeaters are temporarily
connected between two lines and thus serve as connecting devices with the
requisite grade of amplification for the building up of trunk communications
of a more occasional nature.
Repeater stations cannot be under constant supervision but are partially left
unattended. They should therefore be built up on a simple system providing
great reliability of working. This should also be done from the point of
view that fault arising can be rapidly localized and easily remedied. It is
advisable to build the repeater stations entirely mains-connected, the power
being taken from the public mains, a 24-hour battery reserve being provided
to cover any temporary interruption in the mains supply. The repeater station
includes a test equipment, enabling amplification measurements to be made,
as also attenuation and level measurements together with supervision and
control of the repeaters and the ringing repeaters.
Repeaters are of two kinds: two-wire repeaters and four-wire repeaters. The
former are the more frequent, since two-wire circuits are naturally less expensive.
Two wire circuits, however, carry speech in both directions on the
same pair of wires and two-wire repeaters must therefore be two-way. In
order that the speech amplified in one direction shall not return through the
repeater device for the other direction, causing self-oscillation in the repeater
and announcing itself by a continuous howling on the line, the two amplification
directions in the repeater must be connected to the line over a differential
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