LOCO NOMENCLATURE has say 5 parts to it, e.g., WDM3A
1) W = Broad Gauge, Y = Metre Gauge, Z = Narrow Gauge , N = Narrower Gauge (as in some hill railways, Z = 2' 6" and N = 2')
2) D = Diesel, A = AC Electric, C = DC Electric , CA = AC+DC (Dual), B = Battery,
3) P =...
more... Passenger, G = Goods, M = Mixed, S = Shunter, M = DEMU/MEMU
4) Power Rating for Diesels = Number X 1000 HP, For Elec. = Version Number
5) Incremental Power Rating for diesels, for others Variants, Model Variations
(Source : Ravi Bhai in Indian Railways On Facebook )