In message , Paul
Scott writes

I wonder if it is a result of the Desiro 450 order being changed?

I'm sure it is.

Perhaps the other Desiros would have been single class.

Yes - the original plan for suburban services was 2 x five-car units
without any first class accommodation. As I recall, the SRA wouldn't
agree to platform lengthening to allow 10-car trains, so instead SWT
bought more of the four-car units (with 1st class) already ordered for
outer suburban services.

(snip)

--
Paul Terry