If you mean the long term 8 TfW intend to keep, the 8 are already in the fleet.Northern are waiting on 15 x 156 from EMR who are waiting 170’s from TfW who are waiting on 197 / Mk4 / 153’s. These will replace the 153’s (over which only 4 are diagrammed) but I’m not clear if they will also replace the 155’s.
Could TfW requirement for 8 x 153 be met by Northern’s compliant 7 x 155 ?
Any extra 153s would just be to support the current risk associated with the unreliable 769s.
I would think introducing 155s would require train crew training and route clearance, which is the last thing TfW need more of.
I can't see the 170s leaving before the 231s arrive.