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Re: Third Rail
Author: Ernest H. Robl
Date: 08-18-2009 - 19:52
As others have pointed out, there are some serious limitations
to the use of third rail, which is one of the reasons it is
now primarily used for grade-separated transit systems.
Third rail requires gaps in and around switches and at other
complex trackwork. This is less of a problem for EMU transit
equipment, as, while some of the pickup shoes may be in a gap
area, other parts of the train are still receiving power, and
you are unlikely to have a train stranded by stopping with
the engine pickup shoes in a gap.
However, at those gaps, you need to have the third rail dip
up or down (depending on whether you are using overrunning or
underrunning pickup). This incline where the shoe begins to
make contact or lose contact is usually fairly short. Hit it
at a high speed and you are likely to break or damage the third
rail shoe.
The sudden loss of power and reapplication of power is also not
particularly good for the electrical components, particularly if
they are operating under full load.
With overhead wires, you don't have that problem, the pantograph
stays in contact with the wire even through the most complex
track, such as double-slip switches. Yes, catenary does sometimes
have "gaps" -- actually short sections of unenergized wire in
special situations: (1) at low clearance points where the energized
wire would otherwise arc over and (2) at the boundaries between
sections fed by different substations.
In the Netherlands, I also traveled on an electrified branch line
that had a number of small draw bridges over canals. These bridges
were not equipped with overhead wire, as this would have been very
complex and expensive. So, there are linseside signs telling the
engineer to cut power, and drop the pantograph -- coasting through
these gaps.
There are UIC-standard signs for this which are used throughout most
of Europe that instruct the engineer to pull the main breaker and/or to
drop the pantograph (and then reverse these steps on the other side.
These are diamond shaped signs with a white border and a dark blue
background.
They are illustrated and explained here:
http://members.aon.at/sig/asr/ensigael.html
In the following photo, made on a German ICE high-speed line, you can
see one of these signs, though you have to look for it in the low
resolution Web image:
http://www.robl.w1.com/Pix-2/I-911558.htm
This is a junction between the dedicated high-speed line and a feeder line
on which work trains can be brought onto the high-speed line. It can also
be used to detour equipment on and off the high-speed line, if necessary.
The connecting line has a runaway safety track to keep equipment from
accidentally getting onto the high-speed line. That runaway track is
visible to the upper left.
If you look between the left main track and the feeder line, there is a
diamond-shaped sign on a post, with an arrow atop pointing to the left.
That indicates that the unaway track is not electrified to its full
length. Basically the translation of the description of this type of
sign is "equipment with raised pantographs prohibited."
I have a higher-resolution version of this image as the desktop image
on one of my computers. There the "no pantograph" sign is quite easy
to see.
-- Ernest
to the use of third rail, which is one of the reasons it is
now primarily used for grade-separated transit systems.
Third rail requires gaps in and around switches and at other
complex trackwork. This is less of a problem for EMU transit
equipment, as, while some of the pickup shoes may be in a gap
area, other parts of the train are still receiving power, and
you are unlikely to have a train stranded by stopping with
the engine pickup shoes in a gap.
However, at those gaps, you need to have the third rail dip
up or down (depending on whether you are using overrunning or
underrunning pickup). This incline where the shoe begins to
make contact or lose contact is usually fairly short. Hit it
at a high speed and you are likely to break or damage the third
rail shoe.
The sudden loss of power and reapplication of power is also not
particularly good for the electrical components, particularly if
they are operating under full load.
With overhead wires, you don't have that problem, the pantograph
stays in contact with the wire even through the most complex
track, such as double-slip switches. Yes, catenary does sometimes
have "gaps" -- actually short sections of unenergized wire in
special situations: (1) at low clearance points where the energized
wire would otherwise arc over and (2) at the boundaries between
sections fed by different substations.
In the Netherlands, I also traveled on an electrified branch line
that had a number of small draw bridges over canals. These bridges
were not equipped with overhead wire, as this would have been very
complex and expensive. So, there are linseside signs telling the
engineer to cut power, and drop the pantograph -- coasting through
these gaps.
There are UIC-standard signs for this which are used throughout most
of Europe that instruct the engineer to pull the main breaker and/or to
drop the pantograph (and then reverse these steps on the other side.
These are diamond shaped signs with a white border and a dark blue
background.
They are illustrated and explained here:
http://members.aon.at/sig/asr/ensigael.html
In the following photo, made on a German ICE high-speed line, you can
see one of these signs, though you have to look for it in the low
resolution Web image:
http://www.robl.w1.com/Pix-2/I-911558.htm
This is a junction between the dedicated high-speed line and a feeder line
on which work trains can be brought onto the high-speed line. It can also
be used to detour equipment on and off the high-speed line, if necessary.
The connecting line has a runaway safety track to keep equipment from
accidentally getting onto the high-speed line. That runaway track is
visible to the upper left.
If you look between the left main track and the feeder line, there is a
diamond-shaped sign on a post, with an arrow atop pointing to the left.
That indicates that the unaway track is not electrified to its full
length. Basically the translation of the description of this type of
sign is "equipment with raised pantographs prohibited."
I have a higher-resolution version of this image as the desktop image
on one of my computers. There the "no pantograph" sign is quite easy
to see.
-- Ernest
