Running a loco is not "running a train" how many cars behind a loco does it take before it is a "train" 1?...3? Is there a particular number?
See replies above. A "train" can absolutely have zero cars.
Is there a particular ORDER of cars? We all know the caboose goes at the back but what about the "middle"? Is there a "proto" order? Box cars before tank cars etc?
Train ordering is usually based on destination, with cars sorted in yards into "blocks" of cars all going to the same destination (or at least *NEXT* destination, where they may be sorted again for other routes). Within each block, cars are generally not sorted any further, although there are some restrictions in separating different types of hazardous materials to be considered. Sometimes there will be different blocks for the same destination based on type of traffic (e.g. intermodal, automotive, perishables, general) since one type of traffic may be prioritized or sent to a different yard or facility. Type of car is not really considered, except commodity may be. (e.g. ice reefers and stock cars were often handled in specific blocks because the cars' contents are perishable and time-sensiitve and require special handling. Not because there's any effort made to just keep different car types separate. If the cars were moving empty, it wouldn't matter.) Other groupings of similar cars may just occur naturally since a bunch of tank cars picked up at an oil refinery will just travel together until they're split up for different destinations. Same for hoppers from a mine or quarry, boxcars from a paper mill, racks from an auto plant, etc. If a large group travels from one large industry to another, they'll just naturally stay in a group since they're moving together.
Trains may handle one or more "blocks" of traffic.
e.g. a train heading from New York to Chicago will at least have a "Chicago" block, but might also have "Pittsburg" and "Detroit" blocks to set out along the way (just making this up, not sure if there's actually a specific route that travels between all these places).
When the train gets to Chicago, it will be yarded and the cars sorted for other connections, as the "Chicago" block(s) from New York and Detroit may have a mix of cars actually locally destined for Chicago, and cars for other connections via Chicago, interchanging to other trains or railways for the midwest or west coast.