In message 01ca9dfe$3cdf4680$a5d3403e@default, at 22:16:15 on Mon, 25
Jan 2010, Michael R N Dolbear remarked:

Now my draft explanaion of OSIs
===========
As you know Bob, in Britain fares work differently on buses and trains,
on a journey by bus each bus taken is a separate fare,

"in London", perhaps. There are buses elsewhere in Britain with through
ticketing.

but on trains
and underground you are charged a fare for the whole journey even if
you have to change en route.

In the usual case when you never go through an exit barrier until your
journey is complete Oyster's only difficulty in working out the proper
fare on trains and underground is deciding what route you took but, for
example when changing between rail and underground, you will often go
though an exit barrier then a little later go through an entrance gate
to continue your journey.
In some case this may involve crossing the street to a station with a
different name or to a different part of the same rail or underground
station but in all such cases Oyster must recognise that a jouney is
not ending but continuing Thus for this purpose the Euston - Kings
Cross - St Pancras interchange consists of three rail stations and
three underground stations (because of Euston Square) and the Victoria
interchange is two barriered rail stations - Victoria (Southern) &
Victoria (SouthEastern) plus an underground station..

I wouldn't use the Euston/KX example as there are other complications,
and the three ticket halls at KX doesn't help either. The Victoria
example is sufficient.

Or you could make a generic example of "most terminal stations" that
have separate barrier lines for National Rail and Underground; or pick a
less complex station with two sets of barriers (eg Vauxhall). A good
example on the Underground is Hammersmith, with two stations either side
of the road, and fairly easy to find on a map because many readers will
want to cross-check to make sure they understand.

If at one of these interchanges (Out of Station Interchanges, OSIs) you
pass through another entrance gate within the specified time allowance,
say 20 minutes, the journey is regarded as continuing. After that time
or if you touch in on a bus or go to some other station that ends the
old journey.

====

Critics, please remember we want to leave out as much detail as
possible and just get the concept over. I don't want to mention /here/
that Kings Cross (Metropolitan) still counts as yet another underground
station nor that some stations don't have barriers, only validators and
so forth.





--
Roland Perry