The original plan is to move the 175's to another TOC once the 197's arrive, as the Chester depot will be a CAF facility. They could work out of Longsight but the refurbished 175's look so good I wonder if TFW may want to keep them for longer than planned.
There's a lot of logistical problems ahead for TFW as they are getting 77 197's and Chester depot isn't very big & Mach depot isn't that big either. They want 21 ERTMS equipped 197's, so I wonder the remaining 56 units will be stabled ?
There's 51 units between the 158's and 175s alone. Add in the 150's and 153's used on the mainline and you're not going to be far off 70+ units.
Maintenance Depots are different from stabling points. Units will likely still be stabled at the current locations:
Holyhead
Chester station
Crewe
Shrewsbury
Machynlleth
Canton
Carmarthen
Canton will have the ability to store more mainline units once most of the Valleys fleet moves to Taffs Well and Valleys depots.
Not all ERTM's equipped units will be on the Cambrian at the same time, or need to be stored at Machynlleth, 158s are operating all over the network currently.
The 197's will start arriving from Oct/Nov of this year, with the 175 leases due to end on 31st May 2022. That gives about 6 months for the fleet introduction and training to take place until the whole 175 fleet is gone. According to the original Franchise fleet timeline, the 175's were due to start to leave at the end of this year/start of 2022. That seems a stretch given that all kinds of training and familiarisation will need to be done with the 197s, not only train crew but other operational teams. Time is quickly running out, with just over a year until the 175's will all be gone. I can actually see leases being extended, especially if there is any delays to the 197s arriving.