On Operation
70 http://www.ModelRailroader.comBy Jerry DziedzicAuthority and protection
June’s “On Operation”
described how Direct Traffic
Control (DTC) emerged from
timetable-and-train-order
(TTTO) practices. It gave an
example of a meet and offered
some ways to use DTC on a
model railroad. Further study
makes a good lesson in two
fundamental principles:
authority and protection.
Authority permits a train
to move over a specified sec-
tion of main line. Protection
keeps trains (and also people
and equipment) apart,
whether they’re moving in
the same direction or in
opposite directions. Each
major traffic control system,
from TTTO and DTC to
Track Warrant Control and
Centralized Traffic Control
(CTC), satisfies these criteria.
Imagine the danger if
trains entered a main track at
will, without knowledge of
other trains using the same
track or workers not knowing
of its approach. Authority
makes protection possible.
When a train is authorized to
proceed between two points,
it will enjoy protection if
other trains are denied
authority to use that track.
A TTTO dispatcher (DS)
relies on timetable schedules
and train orders to inform
crews of the movements of
other trains. Miles away from
any station, a crew can decide
when it’s safe to go and when
it’s not. Radio made DTC and
TWC possible by giving DS
almost instant communica-
tion with trains, authorizing
their movements. Central ized
Traffic Control uses wayside
signals, not radio, to give
trains movement authority.
Timetable-and-train-ordercontrol demands independent
decision-making of a crew,
sharing responsibility for
authority and protection with
DS. In strong contrast, the
other systems place the full
burden on DS.The track schematic below
illustrates the single-track
main I described in June.
Signs like those in the
Southern Pacific illustration
mark the block limits. Note
how each block extends from
the east siding switch (ESS) of
the previous block to the ESS
of the next. An eastbound
authorized to use the Anna
and Bess blocks can “hold the
main” at Bess but go no far-
ther than the sign marking
the end of the Bess block.
Two trains are shown,
Extra 7859 West and Extra
9288 East. The dispatcher had
already given the Anna and
Bess blocks to Extra 9288
East and the Cloy block to
Extra 7859 West. He then
radioed “Extra 7859 West
with Engineer Jones, take sid-
ing at Bess, over.” The engi-
neer replied: “Extra 7859 West
with Engineer Jones, I am to
take siding at Bess, over.” DS
answers: “That is correct,
out.” These transmissions fol-
low the instructions SP gave
when it introduced DTC in
New Mexico in 1984, still
found in today’s General
Code of Operating Rules.Note that DS has not given
the Bess block to Extra 7859
West. Blocks include only the
main track, not sidings. After
Extra 7859 West pulled into
the siding, a member of its
crew closed up the ESS,
restoring it for movement onthe main. The loudspeaker in
the dispatcher’s office
boomed: “Extra 7859 West,
Engineer Jones, I am releas-
ing the Cloy block, 5:01 p.m.,
over.” DS answered, “Extra
7859 West, Engineer Jones,
you are releasing the Cloy
block, 5:01 p.m., over.” Then
Jones confirmed: “That is
correct, out.” This freed DS to
start a new movement, autho-
rizing Extra 9288 East to pro-
ceed in the Cloy block.
Had DS directed Extra
9288 East into the siding to
set up the meet, its crew
would have released the block
as soon as the train cleared
the west siding switch, not
after it passed the End Bess
Block sign.
These actions satisfy the
conditions of authority and
protection for both trains.
The specific order in whichthey occurred enabled DS to
issue authorities that didn’t
overlap, protecting each train
from the other.How are DTC procedures
simpler than TTTO? It
doesn’t appear so here. Direct
Traffic Control required a
series of step-by-step instruc-
tions where a single train
order, Extra 9288 East
meet Extra 7859 West at
Bess, would have sufficed.
However, TTTO is complex.
I count 11 other train-order
forms that might have applied
to the meet, with more varia-
tions within themselves. The
rule of superiority further
complicates things with right,
class, and direction. Direct
Traffic Control relieves a crew
of this decision-making,
instead having it act on a dis-
patcher’s explicit instructions.
Understanding authority
and protection makes it pos-
sible to devise an easy-to-use
system for your own layout,
whether it adheres to proto-
type methods or not.Southern Pacific created this illustration for a booklet
familiarizing employees with new Direct Traffic Control
procedures. E.O. Gibson collectionEast Siding Switch (ESS)Bess Cloy7859W9288EAnnaWest ESS ESS