• The problem, as I understand it, is that the train operator has to procure special rolling stock, which is a bit of a faff and difficult to accommodate given the parlous state of the network. As Greenbank implies, there simply aren't enough such events to make the necessary faff worthwhile--remember that the train companies don't own the rolling stock and it has to come from quite far away. (This may be wrong, it's just what I remember being told. Train experts, please.)

About