The ICR specification defines two intermediate coach types. B and B1, to the passenger they are the same but under the floor B1 has sanding equipment. (All A coaches have sanding gear)
A 3 car set has no B1 coaches, a 6 coach set has a B1 coach
My understanding, 4/5/6 coach requires at least one B1 coach, a 7/8/9 set would need 2 B1 coaches
A two coach in theory is possible but likely the train management system wouldn't understand that setup, but the seating capacity in that mode would be very low
Obviously its possible to make a B into a B1 given the parts, but no ability to change the fleet exists currently as there are only 15 B1 coaches and 15 6 car sets
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