The common case is the unit diagram looks like this <main depot code><2- or 3-digit number>. Those numbers depending on type of rolling stock (class). SWR class 158s diagram numbers 101 and so on (e.g. SA101). 159s -201 and so on (e.g.SA201). 444s - NT301 and so on (Northam), 450s - NT401 up to...