It used to be 2 diagrams for the Heart of Wales when they worked Swansea-Shrewsbury but many get extended to Crewe and in the South they used to get extended to Cardiff as 'Swanline' services but havent for a while due to the Swanline stops mostly being picked up by the fast trains to Manchester since 2020.
Now with the extra train per day on the HOWL and the short workings at each end id say at least 3 diagrams will be needed. So 3 x 153 pairs with another pair of 153s for maintenance each day.
I assume they are still going to be maintained at Canton which means we could see them still on one Swanline in each direction or an ECS movement will be needed to get them to Swansea.
They need 4 153s at the moment
153 - 1
2V00 0419 Shrewsbury to Llandrindod
2M02 0550 Llandrindod to Shrewsbury
2V06 0856 Shrewsbury to Swansea
2M10 1418 Swansea to Shrewsbury
5M10 1837 Shrewsbury to Shrewsbury Abbey Frgte Cs
2V14 2129 Shrewsbury to Llandrindod
2M16 2310 Llandrindod to Shrewsbury
153 - 2
2M04 0547 Swansea to Shrewsbury
2V08 1121 Shrewsbury to Swansea
2M12 1819 Swansea to Shrewsbury
153 - 3
2V04 0522 Shrewsbury to Swansea
2M08 1140 Swansea to Shrewsbury
2V12 1807 Shrewsbury to Swansea
2B70 2230 Swansea to Cardiff Central
153 - 4
2M06 0909 Swansea to Shrewsbury
2V10 1517 Shrewsbury to Swansea
2M14 2115 Swansea to Llandovery
2V16 2243 Llandovery to Swansea
5V16 0025 Swansea to Cardiff Canton Sidings