Other than passing through to or from Wolverton it is unusual these days to see a 166 at Reading, but it looks like 166207 is working this evening Bourne End diagram.
2B42 1714 Maidenhead to Bourne End
Realtime Trains | 2B42 1714 Maidenhead to Bourne End | 17/05/2024
Real-time train running information for 2B42 1714 departure from Maidenhead to Bourne End on 17/05/2024. From Realtime Trains, an independent source of train running info for Great Britain.www.realtimetrains.co.uk
I guess it worked up from Westbury earlier?
5R60 1115 Westbury to Reading Traincare Depot
Realtime Trains | 5R60 1115 Westbury to Reading Traincare Depot | 17/05/2024
Real-time train running information for 5R60 1115 departure from Westbury to Reading Traincare Depot on 17/05/2024. From Realtime Trains, an independent source of train running info for Great Britain.www.realtimetrains.co.uk
Not a scheduled fleet changeover; will probably be undone again over the weekend, but 166207 needed to come out of service this lunchtime for fuel etc; and there was nothing available in the “West” Turbo fleet to replace it at Bristol; meanwhile Reading had a unit spare and a driver to take it to/from Westbury; so best use of resources available has resulted in the temporary swap. 165120 is the “East” unit that has gone West.
Looks as though with 166207 available for evening service, Reading have sent it out on a diagram that doesn’t see it go too far afield.