Discussion Groups / English Usage / January 2004

Ray Heindl:

Don Aitken:

Right.  Of course, other layouts are possible, but that's the way the
Underground does it.  See e.g.

  <http://www.davros.org/rail/photos/woodlane/midsize/wlane113.jpg>
  <http://www.davros.org/rail/photos/woodlane/midsize/wlane132e.jpg>

The center rail is nominally at -210 V, the outside one at +420 V.
In the second picture you can see the positive rail changing from one
side of the track to the other.

Historically, the C&SLR (1890, now Northern Line City Branch), W&CR
(1898, now Waterloo & City Line), and CLR (1900, now Central Line), all
originally used only a third rail, each line putting it in a different
place; the GN&CR (1904, later the Northern City Line, then the Northern
Line Highbury Branch, then BR's "GN Electrics" route, now served by
WAGN) used third and fourth rail, but put them outside the running rails
on both sides.

When the Metropolitan (1863, now the Metropolitan and Hammersmith & City
Lines) and the Metropolitan District (1868, now the District Line)
converted (except for outlying bits) from steam to electric trains in
1905, they had to choose a common system since they had a number of
shared routes, most notably the Inner Circle (now the Circle Line).
The Metropolitan wanted to use a Hungarian 3-phase system requiring
two overhead wires *and* return current through the running rails; the
District wanted the fourth-rail system that was finally chosen.

The BS&WR (1906, now the Bakerloo Line), GNPB&R (1906, now the Picca-
dilly Line), and CCE&HR (1907, now the rest of the Northern Line) were
owned by the same company as the District, so when they opened, they
used the same system.  This made it clearly the dominant system on
the underground lines, and after they were nationalized and unified
as the Underground system in 1933, the others were converted to the
common system.  (The W&CR didn't join the Underground until 1994; it
was converted to the Southern Railway third-rail layout in 1940, then to
the Underground system in 1992, each time when new trains were put on.)

It's not the rails themselves, but other things nearby, especially
the metal lining of the tunnels.  The issue is galvanic action corroding
the metal where stray current leaking from the running rails into the
ground passes from one material to another.

An additional benefit of the 4-rail system is that it tolerates a short
circuit to ground at any one place.  If the positive rail is shorted to
ground, the negative one simply drops to -630 V, and the trains keep
running, and the problem can be fixed when convenient.  Unless there is
a fire, of course.

Harrumph -- that's 11 feet 8 1/4 inches!  Well, except for the post-WW2
tunnels, where there's a bit more width.

Right.  And since the normal voltage on each rail is lower, there is
also a slight improvement in safety.

The nominal voltage for the third-rail system is 750 V these days,
but the trains can get by on a lower voltage that won't burn out the
Underground trains, so it works.

Incidentally, the old Nord-Sud company in Paris (which built what is now
Line 12 of the Metro, and part of Line 13) used to run its trains with a
third rail *and* an overhead wire, one at +600 and the other at -600 V.
This was not used to achieve 1,200 V through the motors, though; the
trains had two powered cars, one drawing from each supply and both using
the running rails for return current (which therefore most of the time
mostly just flowed from one power car as far as the other).