All the employment on-costs that apply to staff - final salary pension, employment taxes, additional depot space, that sort of thing - it all adds up. Overtime just attracts a small amount of additional NI.
But a reliance on overtime and/or rest day working demonstrably prevents
GWR▸ from running a half-way decent service leading to the public using other forms of transport, or not travelling at all, and a consequential significant loss of revenue.