Off-peak diagramming / unit usage question

[nw1]

Was just thinking of a potential conundrum in unit diagramming.

Let's say a small network requires 25 units in total, including those used only in the peak.

There are two ways of diagramming the off-peak service. You either have 10 units in service off-peak with shorter turnaround times, or 12 units in service off-peak with longer turnaround times.

Let's say platform occupancy isn't an issue, i.e. there is no platform occupancy limitation to prevent the 12-unit solution.

Is there any particular advantage in minimising the number of units out off peak, and going for the 10-unit solution? Mileage-wise both would be equivalent, you either have 10 units on a high mileage per unit or 12 units on a slightly lower mileage per unit, but total mileage the same. And 13-15 units are only used in the peak in any case.

Let's also say the units are cycled so you don't get the same 10 or 12 units out on off-peak duties each day, i.e. each unit gets a number of peak-only days.

So if the 12-unit solution involves some advantage, such as slightly longer turnaround times (and thus increased reliability) or simpler diagramming, is there any good reason to not choose it over the 10-unit solution?

 

[87015]

Driver/Guard cost if you are keeping them with units. Not much point stepping units down and stepping crew up! A mix of higher and lower mileage diagrams tends to be helpful to fleet as well for getting stuff to the correct mileage for exams etc.

 

[endofthetunnel]

Having some units ‘spare’ during the middle of the day allows them to go for a trip to the depot for fuel/service etc, and then able to be stabled out somewhere overnight ready for the start of service.
It also gives some resilience if there’s unit issues in the morning to do something about it before the evening peak.