Dave Arquati wrote in message ...

You said that "if Shenfield is only served by CrossRail, then at
Livepool St most of the passengers will want to get off, adding to
passenger congestion."

It's a fair point but as you say, Liverpool St's capacity will be
increased. Crossrail's design will take into account the fact that a
large proportion of passengers will disembark at Liverpool St, so
congestion (I assume you mean on the Crossrail platforms) should not be
a problem. These are *big* stations - don't forget there will also be an
exit at Moorgate, so some Shenfield line passengers will use Liverpool
St Crossrail but not Liverpool St itself, if you see what I mean.

Yes, but the I would expect the bottleneck to be on the train itself,
as all the passengers arriving at liverpool St from other lines now
try and get onto CrossRail trains at liverpool St, just as 3/4 of the
train is trying to exit. With all trains going to just two locations,
I'd expect 50-75% of the train contents to change (ie exit and enter)
at Liverpool St, increasing the platform time and reducing the
throughput.

A solution would be, as in Munich, to have double sided access.

However, the problem to note is not so much that a majority of
passengers will disembark at Liverpool St, but that running all
Shenfield trains as Crossrail reduces operational complexity, which will
in turn increase reliability and allow a high frequency to be achieved.
If some terminate at Liverpool St mainline, this increases the number of
movements across a junction which could cause problems.

Agreed, that's the trade-off. Most of the other lines would join at
Stratfiord, so one flyover there would ease this particular problem.

snip

Five branches means huge operational complexity and extra cost - unless
they all have flying junctions then that adds conflicting movements,
which means reduced reliability.

Not all, but only near the "root" - probably one at Stratford.

How do Grays trains get to Crossrail? There are a number of options but
they are either expensive or reduce reliability on existing lines. For
example, they could run from Stratford to Barking via Woodgrange Park -
but so do a large number of freight trains from Tilbury, which all have
to cross flat junctions. Or they could surface near Bromley-by-Bow
instead - but that means extra tunnelling and an underground flying
junction. Similar arguments may apply to any other branches.

*This makes 20 tph, compared to tunnel theoretical capacity of 24 tph.
I would then have train waiting at Liverpool St and Paddington to fill
any spare slots caused by late arrivals. These would only go Liverpool
St to Paddington, and make up the numbers to 24 tph.

This would require turnback capacity at Liverpool St (which will be more
expensive) and extra platforms at both Paddington and Liverpool St
(which will also be more expensive - particularly at Liverpool St which
I think will be bored).

Also, how do you decide when an arrival from the surface routes is late?
Say trains must be separated by at least 75 seconds, and each "slot"
is 150 seconds (2.5 minutes = 24tph). If the previous train departs into
Central London right near the end of its slot, then the next train
cannot depart until 75 seconds into the slot.

Once those 75 seconds have passed, how many of the next 75 seconds do
you wait before the due arrival is considered "late", given that it will
also take a reasonably "long" time between deciding to despatch a train
and it departing the station, given that passengers have to realise that
a train is about to depart and to board it.

This all depends on being able to predict the lateness a few minutes
before it happens. With some smart IT and train positioning systems,
this should be possible.

What happens if two consecutive trains are late? You won't be able to
get a new train ready in 150 seconds, e.g. at Paddington - even if the
new train is waiting in turnback sidings at Westbourne Park and departs
for Paddington as soon as it is decided to despatch the standby train
already in Paddington. In fact the second late arrival may already have
passed Westbourne Park.

If you don't have it, you get bunching. A late train is a more used
train, which increases platform time, which makes it later. On buses,
this is solved when the following, empty bus overtakes, and hoovers up
all the waiting passengers. On CrossRail, a fast response insertion
would do this.

Although I like the idea of standby trains, the more I think about it,
the more it seems unfeasible for a high-frequency service.

Depends on being able to have 3 minutes warning of lateness (or a 5
minute gap)