The replacements are in service, around 10 350/2s were able to go off lease when 730/0s were introduced on the Walsall-Wolverhampton and Chase Line services, and a further 2 will have been displaced by the pair of 730/2s diagrammed in the week. A further 2 730/2s are expected to enter service on an all day diagram on Monday.
There’s been lots of disruption these last two days so it’s natural that units will have been displaced.
It isn't just today that 4-car short formations are and have been happening, it's been a daily occurrence for months. Putting it down to disruption over the last couple of days is just glossing over it and sweeping it under the carpet.
Only 1 pair of 730/0's sees use on the MK / Tring circuit on Saturdays, again this has been the case for months. 3 or 4 if not more pairs sit idle on Bletchley, while at the same time, numerous 4-car 350's are flogging up and down on not just the MK / Tring locals (where they are arguably and occasionally just about acceptable) but also on Birmingham and Crewe circuits where they are absolutely not.
Some of the off-lease units in Wembley haven't turned a wheel since before Christmas, simply because the DfT / railway (delete or insert other as appropriate) isn't interested in providing a service to the passengers, purely extracting more and money from them to satisfy its own greed.