In message
-sept
ember.org, Recliner wrote:
Tube lines have fleets of identical trains, you can't just order a few
more later.

Not always, they don't. Even the Victoria Line used to run a mix of 1967
and 1972 Tube Stock.

But in this case, the 73TS were built while the Heathrow extension was
under construction, and a large enough fleet to cover it was ordered.
Without that extension, the order would have been for a significantly
smaller fleet.

Actually, I don't think so. The order was for 87.5 trains. I reckon that
the extension only needed 4 of those.

Let's do a finger-in-the-air test. It takes about 85 minutes from
Cockfosters to Heathrow Central and 94 to Uxbridge. The last notes I
have say that each of these gets 12 trains per hour in the peak (5
minute spacing). Assume that at each end a train departs as one arrives,
so there's always one at the terminus. That would mean (85+5)*2/5 = 36
trains for the Heathrow branch and (94+5)*2/5 = 40 for Uxbridge
(probably actually 38 or 39 because some terminate at Ruislip). That's
about 75 trains. The rest allow you to thicken the service in the peaks
and provide spares.

Ah, another source says the current service uses 76 trains on Saturdays
and 68 on Sundays.

So the stock order was very probably based on that 12 tph figure.

Hounslow West to Heathrow Central will be 7 or 8 minutes of those 85, so
the extension adds 15 minutes to a round trip. So without it we'd need 3
trains less. Plus a fourth as a margin. QED.

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