Firstly, thanks to you both for the explanations.

On Wed, 25 May 2005, Mark Brader wrote:

Tom Anderson and Clive Feather wrote:

The discussion says that something called a '4-track relay terminal
with a 2-track relay' used to exist at Park Row on the New York subway.
No idea what that is, but the poster seemed to be impressed.

I asked a knowledgeable friend.

Firstly, it wasn't on the Subway but on the erstwhile Brooklyn Bridge
Railway, at the Manhattan end. Turning it into UK terms, the layout
would be:

####D####
/--------------\
|------* ####A#### \
\-----------\ /--*----
X
/-----------/ \--*----
|------* ####D#### /
\--------------/
####A####

The platforms marked D were for departing passengers and A for arriving
ones.

Wow. I can't even begin to figure out what the capacity of that would be!

Do trains drive on the left in the US, then?

No: remember Clive "turned it into UK terms".

Doh. Sorry Clive!

I was Cc'd on that email thread, and it turns out that the above
description may not be exactly right. My interpretation of the
somewhat contradictory information is that the layout actually
worked *this* way:

/--------------\
|------* ######### \
\ /--------------o---------
X
/ \--------------o---------
|------* ######### /
\--------------/

This gives separate arrival and departure platforms, and also makes
it impossible for two trains with passengers aboard to collide
head-on on the diamond.

That seems like a much better arrangement.

As a further safety aid, the tracks at the right were actually
gauntleted (interlaced), with two pairs of rails that diverged
(if my interpretation is right) at the positions o. That is, in
terms of individual rails, the layout at each o was:

-------------------------------------------------
/---------------------------------
/
-------------+-----------------------------------
/ /-----------------------------
/ /
/ /

And if a particular train started from the south face of the departure
platform, then it would use the south face of the departure platform all
day, and also the south face of the arrival platform, staying always on
the south rail of each pair.

So each track was in fact four rails, of which only two were in use at
once? There are two logical tracks sharing the same space?

One more source of danger elimnated.

How so?

The X is a simple diamond crossing without slips.

Is the entirety of railway terminology invented purely to wind me up?
I'm guessing a diamond crossing is just where two pairs of rails cross...

Uh-huh.

what's a slip?

A slip is where one of those is combined with points so the train can
change from one line to another *or* go straight across. It's expensive
to build and maintain, and therefore normally used only where there
are space constraints.

You mean not enough to build a proper curve? Proper curves being cheaper?

The usual kind is a double slip, with two pairs of curved rails and four
sets of points:

http://www.iwsteamrailway.co.uk/pages/Civ_engineering/photos/Track%20Pack%20Apr%2004/TP%20Apr%2004%205.jpg

A single slip has only one pair of curved rails, allowing 3 rather than
4 moves in each direction. Near major terminal stations, a series of
slips is often used to form a layout allowing a train to cross (in one
direction, say to the left) from any one to any other of a set of
tracks. This example
http://wvs.topleftpixel.com/photos/union_rail_tracks.jpg is the
western approach to Union Station here in Toronto; note the single slip
near the white post as well as the double slips.

Got it.

Is there some sort of encyclopedia of railway engineering that i could get
hold of which would save you from these questions?

tom

--
The revolution will not be televised. The revolution will be live.