If every train on the Manchester to Preston corridor is made up of 4 x 20metre carriages then I don't think that there is an essential need for six carriage formations.
The peak time services on that corridor now are mainly either 2 x 150 formations or 2 x 185 formations and some of the 2 x 150 formations on Hazel Grove services are overcrowded.
I appreciate the Manchester-Preston passengers will have the class 350 via Wigan option but that's only hourly and a lot of people travel between Bolton and Manchester. Even 156s in the off-peak period can be overcrowded on that section if they are in single formation.
Like North TPE, the Bolton corridor needs extra carriages now, not in a couple of years time and replacing 2 x 150s with a single 319 will not be good enough.
And why would the technology used in this latest electrification scheme be so startlingly different to that which has been used in every other 25kV overhead electrification scheme in the past fifty years?
Do you recall the renewed underground track in Liverpool city centre a few years ago? The 507s and 508s needed wheel modifications to be able to run on it, which led to many cancelled and short formed services after the works were supposed to have been completed.
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However I still maintain that the north should be moved to an 8 car railway.
I'm not convinced at the present time we should make making platforms at stations like Blackrod long enough to take 8 carriages when there's a greater pressing need for investment on other lines. 6 carriage trains or SDO seems like a better short term solution to me.