The 3 B2s were the original sighting, and there was 2071 southbound with
2107 followed by 2017 northbound, and yes there are plenty of other
combinations but to have these 3 from 2 different depots in the same
location given some of the moves recently just really freaked me out. Not
being a dedicated follower of depot allocations I was unaware that this was
even possible. I suppose if one had the time it would be possible to work
out what other 3 digit combinations in the one street were possible based on
depot allocations.

Cheers

Hayden

-------Original Message-------

From: Roderick B Smith
Date: 13/01/2008 8:54:19 PM
To:TramsDownUnder@...
Subject: [TramsDownUnder] Re: Numbers

The CTR is trivial: there are six sequences in which they can operate,
in three reversable sets: 7, 8, 9 in one street becomes 9, 8, 7 after a
shunt; likewise 8, 7, 9 reverses to 9, 7, 8; 7, 9, 8 reverses to 8, 9,
7.

The other question is unclear: do mean specifically 2017, 2071 & 2107,
or do you mean all such trios?
eg 2011, 2101, 2110
or 2012, 2021, 2120 (toss in 2102)
and more

Going for just the trio which you saw, and not worrying about the order
in which they were travelling.
There are 130 B2s. If the three are assembled at random from the 130,
then there are 130C3 trios, calculated as 130!/(127! x 3!), or more
comprehensibly (130 x 129 x 128) / (3 x 2 x 1). One of these will be
your trio, so you have about one chance in 360 000 that a trio seen in
a street will be this trio.
However, the three are not necessarily assembled at random.
If they are seen in one street, it is likely that they come from the
one depot. I don't know the depot rosters. If the three come from a
depot with 40 Bs, the chances of having them together are improved:
(40 x 39 x 38) / (3 x 2 x 1) trios, so now one chance in 10 000.
You can do your own calculation for any other depot size.

The easiest case: a depot which has just 3 trams, being these three.
If ever you see three together, it just has to be this trio, and the
pattern in the formula shows this: (3 x 2 x 1 ) / (3 x 2 x 1), just one
trio.

If the street is shared by two depots, one of which has 30 Bs and the
other of which has 15, then use the combined fleet of 45 in the formula.

The whole success of Tattslotto is based on people's misconception of
the true odds.

Regards,
Roderick B Smith
Rail News Victoria Editor

"Hayden Ramsdale" <on_touragain@...> wrote:
> In Spencer street I saw 2071 passing 2107 which was followed by 2017,

> then down near the Casino I saw the 3 restaurant cars heading south in

> numerical sequence, 939 following 938 which was following 937.


> I wonder what the chances are of such numerology being repeated?