A recent Modern Railways article suggests that GWR do not think the aluminium bodied DMUs (classes 158, 165, 166) will be able to survive beyond 2032 due to corrosion and possibly parts. I guess this also applies to 159s.
Sounds like GWR and DfT not in same place with replacement timeline. With GWR wanting BEMUs ordered sooner (and some bits of electrification around Yate, Bristol Temple Meads and Westbury, and towards Oxford), whilst DfT is in more of a not for a few more years mindset.
Also suggests GWR need longer or more frequent IETs in some areas, so need to free up those stuck covering on regional routes.
Meanwhile DfT did update their procurement spreadsheet 11th September and GWR entry now suggests could be additional trains of same type order for other operators
| Procurement of rolling stock for Thames Valley / North Downs and West (including options) | Replacement of all existing DMUs (and possibly EMUs), excluding the IET fleet, with Battery Electric Multiple Units (BEMUs) and/or diesel multi-mode units. Potential for this procurement to include units for other non-GWR routes. |
www.gov.uk