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Today 22 of 24(23) 458s out :D All booked workings plus
Extra pair replaces 455s on Reading curcuit and after midday out of Wimbledon single 45835 on Shepperton curciut.
The common case is the unit diagram looks like this <main depot code><2- or 3-digit number>. Those numbers depending on type of rolling stock (class). SWR class 158s diagram numbers 101 and so on (e.g. SA101). 159s -201 and so on (e.g.SA201). 444s - NT301 and so on (Northam), 450s - NT401 up to...
Midday passenger flows lesser than peak ones, so it`s quite justified. Southern possibly would like to reduce the units` mileage and obviously some maintenance required for the units. All trains cannot be serviced during the night and the depots are not large enough to accommodate all trains...