I think people may see this as a bad thing since a 185 has just 154 standard seats and even if the first class area was replaced by standard seating you still wouldn't have as many seats as you do today with 4 car 158s (292 seats based on porterbrook's numbers) so unless you want to lengthen all the platforms on the route to take 6 cars then 185s aren't a good option for this busy route.
Network Rail's CP5 plans include platform lengthening to allow:
6 x 24m car length at Liverpool South Parkway Up and Down fast platforms, Widnes, Warrington Central and Newton-le-Willows
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Although there's a distinct possibility of replacing 2x 158 and 1x 185 with 3x 185 per hour... 20 minute express frequency seems to be on the agenda...
3tph (express) between Manchester and Sheffield via Stockport is being considered. However, 3tph (semi-fast) between Liverpool and Manchester via Warrington isn't and it wouldn't work unless some of the smaller stations had a reduced frequency of service. The Northern/TPE consultation document suggested Sheffield would have direct trains to at least two out of Liverpool, Manchester Airport and Chester.
If you try to do 2tph between Liverpool and Sheffield and 3tph between Manchester and Sheffield then you won't get a nice clockwork timetable. The timetabling also depends if Selby-Hull is a CP5 electrification project. If not I'd guess at North/South TPE being:
* Hourly Liverpool-Warrington-Piccadilly-Hull (semi-fast diesel)
* Hourly Piccadilly-Selby (semi-fast electric)
* Hourly Liverpool-Warrington-Piccadilly-Sheffield-Nottingham (semi-fast diesel)
* Hourly Chester-Warrington-Sheffield (semi-fast diesel)
* Hourly Manchester Airport-Doncaster-Hull/Cleethorpes (semi-fast diesel)
* Half-hourly Liverpool-Newcastle via Victoria (express electric)
* Half-hourly Airport-York via Victoria (express electric)
That way you can have a 20 minute frequency between Piccadilly and Sheffield and a 30 minute frequency between Piccadilly and Liverpool via Warrington.
If you electrify Selby-Hull then maybe terminate both the Hull and Doncaster and beyond services at Piccadilly and run a Liverpool-Warrington-Airport service in the gap left. Alternatively, if there's a path maybe run a Warrington Bank Quay-Hull electric service.
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Why reform the 155s when you can couple two 153s together where required & avoid the expense of conversion.
It could be cheaper to reform them - the redundant cabs don't have to be removed. If they remain as 1 carriage trains each individual 153 must meet the accessibility standards.
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convert the other toilet space
Non-accessible toilets can remain on 2 car train if there's an accessible toilet elsewhere on the train. However, there's also a requirement for all train toilets to be retention toilets by the end of 2019.