David H Wild wrote:

A big problem is that there are platforms only on the slow line at
Queens Park so, if trains are booked to call there, operating
flexibility is reduced significantly.

There are fast line platforms at Queen's Park but nothing is booked to use
them, nor has there been for some years. Silverlink County services have
used them in emergencies and IIRC some late night trains have stopped there
on occasions, usually when engineering work has been taking place.