Such maps can be misleading. The only service from Shrewsbury towards Birmingham that extends beyond Birmingham New Street/International is the daily Avanti service to London. WMR only operate Shrewsbury-Birmingham New Street services.
For a driver to be allowed to drive a train, they must have received training in both the traction (the type of unit/locomotive used) and the route. In most cases they will have to have driven that traction, and driven over that route, within the previous 6 months.
Route knowledge involves drivers learning things such as the name and location of all the stations on the line, the names and arrangement of the tracks, the type of signalling system used, line speeds, platform lengths, gradients, tunnels, bridges and other notable features. Basically they need to know the route like the back of their hand, to the extent that they know exactly where they are and where they need to start braking, even if it's night time and there is thick fog.
Obviously those requirements, as well as limits on shift length, breaks etc. amongst other factors, dictate what it makes sense for drivers at different depots to be trained on.
Therefore, WMR Shrewsbury drivers are only taught on the route between Shrewsbury and Birmingham, with a diversionary route via Bescot. The guide at the start of this thread illustrates this as follows:
The only other drivers at WMR who have knowledge of the line to Shrewsbury are Wolverhampton drivers:
Hope this helps?