Formation
Also: coaches, carriages, how many coaches
How a train is made up: which units or carriages, how many, and in what order.
A formation is the train as it actually runs that day: one five-car unit, two coupled together to make ten coaches, and so on.
Coaches are usually lettered, A, B, C, which is what your seat reservation means by “Coach C”. The letters on your ticket are about the train that day, not painted on for life.