Depends on what the train does.
Does it carry mail? You will need a lockable storage-mail car. Essentially, a baggage car.
Does it pick up, drop off, and sort mail? You will need a car with a “postal apartment”. These come in lengths of 15’ 30’ and 60’.
You can combine the two above operations in a baggage-mail car.
If there isn’t much storage mail, it might be carried in the postal section. Then the baggage section can be used for baggage. You cannot use a storage mail section for anything other than storage mail.
There may also be separate express cars. I’m not knowledgeable about express. So I don’t know a lot about who did and who didn’t and how it was handled. Seems to me a couple of express packages might go into the baggage section.
And then there’s baggage. Kind of self explanatory.
And coaches. Also self explanatory.
But, in addition, you MIGHT have food service. As in a diner, but more likely a cafe section of a general-use car.
Also, some of the higher end trains might have a lounge area.
All that said, the shortest train you would likely have is one car: a combination baggage-coach.
If you’re into the mail business, a likely small train would be a baggage-mail and a coach.
From there, you can keep adding cars and partial cars until you have a huge train.
Ed