Max, we don't but history has proved the worth of the second standby. Until you get a major incident you don't know what you need.
The Greyrigg (Cumbria) accident is a suitable case to examine for appropriate emergency response - the actual number injured (and one fatality sadly) was very low but the emergency response was huge, assuming that a train with up to 200 people leaving the tracks at 95 mph would produce carnage on a fairly major scale.
No-one has suggested cutting police fire and ambulance crews in the area because they weren't needed.
So, do you plan for the contingency or hope it never happens - over to you Govt....