2-8-2...
It really depends on the era you are modelling. From your references to mail and milk, I think you are looking at sometime ago...!
Passenger service has taken all kinds of forms over the years. Mixed trains and "milk runs" were common on branchlines, where there may have been a stop every few miles. The mixed train may have even done some switching while passengers (who could not have been in a hurry) waited in the combine. Depending on the road, they may have even ridden in a combine "caboose". Milk and other perishables might have been along for the ride as well. In those days, you might well have been able to get on and off where you wanted simply by making a request, and/or flagging down the train.
Passenger operations also may have been carried out by gas-electrics (doodlebugs), railbus, or even railcar. Of course there are the famous high-speed "named" trains as well, especially in the Golden Age of rail travel - the NYC's streamlined 20th Century Limited is one of my favourites.
There are also today's commuter railways that may have their origins in the original "radial" railways that spread out from a central business area like spokes on a wheel (hence the "radial" name).
Also subways, streetcars, and trolleys... The list goes on!
I think though for the era you seem to be thinking of, there was quite a lot of work that went into servicing the passenger trains. They would need to be turned, as the order of the cars was important, and they often had an observation or park car at the end that was "unidirectional". They also needed to be cleaned, loaded with water and food, etc, etc. At certain points, the engine and/or crew might be changed, and supplied refreshed. Also, some trains were split part way through their journey.
I was not a big fan of passenger ops originally, but the more I learn, the more I like it!
Hope that helps.
Andrew