On Tue, 24 May 2005, Clive D. W. Feather wrote:

In article , Tom Anderson
writes
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.

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

I've just realised how to build a terminal with arbitarily high capacity,
provided you don't mind making your passengers choose between an equally
arbitrary number of platforms:

+-[--------+-]-\
### / [ ### / ] \-----
-----+ [ -----+ ] /----
\ [ \ ] /
+-[--------+-]-/
n

Where the bit in square brackets with an n at the bottom is a repeated
unit (think polymers!). Trains come in from the east (and why do trains
always come in from the east in these things?), run along the road at the
southern edge of the structure, then pick a bay to stop in, run in on the
diagonal approach road, get in, stop, exchange passengers, then pull out
on the diagonal departure road, joining the main road at the northern edge
and heading back out east. The point is, there are no conflicting
movements, and no contention for anything except the running roads, so the
terminal doesn't restrict capacity below that which the line supports
(provided you can do the diverges and converges perfectly). Note that when
n = 0, this is a normal single-track reversing terminal, and when n = 1,
it's rather like a Sao Paulo terminal (but with more irritating platform
layout).

I think you have to be rather clever about the order in which bays are
used to preserve even intervals between trains, though.

If you replace the reversing bays with through lines, you get a
multi-track loop:

/-[--+-]-\
/ [ / ] \
|# [ |# ] \-----
|# [ |# ] /----
|# [ |# ] /
\ [ \ ] /
\-[--+-]-/
n

Which is wider, shorter, doesn't reverse the trains and is amenable to the
use of island platforms.

Probably not the most sensible use of railway space, either way!

tom

--
an optical recording release. copyright digitally mastered. .,