This is an extract from a Facebook post by Stourbridge Line User Group following a meeting with West Midlands Railways. The full report goes into both the 172s and the 196s but I have just extracted what I think is relevant on the thread.
Class 196. West Midlands had 12 2-car Class 196/0 and 14 4-car Class 196/1. Unfortunately, six of the Class 196/0 units have been requisitioned by the Department for Transport to operate the new Oxford to Milton Keynes (East-West Rail) service as the previous Government sanctioned the building of the line but not any new trains to operate it. This has coincided with engines on the Class 196s reaching the point where they need replacing, which is causing shortages of available units. West Midlands Railway is in contact with CAF re the supply of replacement engines.
It has meant that Class 172 units are sometimes being deployed on New Street to Hereford services, exacerbating the short-form problem on Snow Hill lines. In fact, I saw a 2-car Class 172/0 unit at New Street yesterday, replacing a 4-car Class 196 on the 1550 service to Hereford.
This gives the answer to a question I asked in a post above. DfT took the 6 Class 196s from the WMT order leaving WMT short of their planned stock.