Having a quick look, I just used and prepared the 458/5 that sits in Clapham during the day, this forms 5U41 which leaves Clapham at 1356 and is booked to arrive at Waterloo at 14:13. I don't know what it then works but 2U41 is the 1428 Waterloo to Windsor which then forms the 1658 Waterloo to Windsor? So whether there's an attachment to be made at Waterloo To make this a Ten Car I'm not 100% sure, maybe the train ID's are just coincidental.
Taking a look at RTT and 5U41 and 2U41 both use platform 16 so it would appear to be the same set. On the natural 2.5 hour cycle of the Windsors the 1428 should do 1658. The only way it seems to me that the 1428 could attach to another set is if the 1407 Waterloo - Waterloo leaves a set behind on the blocks (also platform 16). If it doesn't and the 1428/1658 to Windsor is a 5-car then it would imply that this turn is booked 5-car because again there are no Windsor trains on the Train Formation Updates (at least SWT are consistent in this regard). But the same as last night, there are still another two 458/5 diagrams short formed
16:37 London Waterloo to London Waterloo due 18:02 Will be formed of 5 coaches instead of 10.
18:15 London Waterloo to London Waterloo due 19:41 Will be formed of 5 coaches instead of 10.
16:45 London Waterloo to London Waterloo due 18:11 Will be formed of 5 coaches instead of 10.
Unless there are other trains booked to run with more 458/5 than there used to be, then that looks like only 29 out of the 35 units are available for service (I presume 458501 is still Testing, so doesn't count as part of the fleet yet)
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I thought that the 1707 Waterloo - Hounslow loop and 1843 to Shepperton were booked for a pair of 458/5 but this evenings SWT Train Formation Updates implies that these trains are now normally booked 8 coaches, so a pair of 450 or 455. The comment about lack of toilets suggests that they would normally be present, so booked 2*450?
At least tonight SWT do show the 1707/1843 booked for 10 coaches, even if they have failed to achieve that number.
17:07 London Waterloo to London Waterloo due 18:32 Will be formed of 5 coaches instead of 10.
18:43 London Waterloo to Shepperton due 19:28 Will be formed of 5 coaches instead of 10.
One of the Windsor diagrams is shown as 5 vice 10, but not the diagram that includes the 1658. I wonder if one of the Weybridge diagrams that used to be 5 cars in the peak is now 10? (eg 1622 or 1652).
12:23 Windsor & Eton Riverside to London Waterloo due 13:19 Will be formed of 5 coaches instead of 10.
13:28 London Waterloo to Windsor & Eton Riverside due 14:22 Will be formed of 5 coaches instead of 10.
14:53 Windsor & Eton Riverside to London Waterloo due 15:49 Will be formed of 5 coaches instead of 10.
15:58 London Waterloo to Windsor & Eton Riverside due 16:52 Will be formed of 5 coaches instead of 10.
17:23 Windsor & Eton Riverside to London Waterloo due 18:19 Will be formed of 5 coaches instead of 10.
18:28 London Waterloo to Windsor & Eton Riverside due 19:23 Will be formed of 5 coaches instead of 10.
19:53 Windsor & Eton Riverside to London Waterloo due 20:49 Will be formed of 5 coaches instead of 10.
20:58 London Waterloo to Windsor & Eton Riverside due 21:52 Will be formed of 5 coaches instead of 10.
22:23 Windsor & Eton Riverside to London Waterloo due 23:19 Will be formed of 5 coaches instead of 10.
