You will need to decide what types of trains to run on your layout. There are four types of train that I will discuss: passenger trains, express freight trains, local freight trains, and mixed trains. You will also need to decide on a maximum length for your trains.
Passenger trains consist of one or more locomotives and one or more passenger cars. A passenger train may also carry a combine for luggage or small freight loads or a mail car. A local passenger train will stop at every station, while an express passenger train will only stop at selected stations. A passenger train may switch some cars at an interchange with another rail line. Here, passenger cars are moved from one passenger train to another. Passenger trains offer the least opportunity for car switching of all train types. Express passenger trains have highest priority. All other trains must clear the line for them. Local passenger trains have priority over all other trains except express passenger trains and, perhaps, express freight trains.
Express freight trains generally run from one end of a line to the other, stopping to switch only at interchanges with other rail lines. If the two ends of the line are also exchanges, then express freights have the same opportunities for switching as passenger trains. However, since express freights drop off all cars and pick up all new cars at the ends of the line, they may have more opportunities for switching than passenger trains. Express freights usually have priority over all other trains except express passenger trains.
Local freight trains switch every town and siding they pass. These trains provide maximum switching opportunities. Local freights take priority over no other trains. They must clear the mainline to make way for all passenger trains, express freights, and mixed trains.
Mixed trains are usually local freight trains that also include a passenger car at the rear of the train. They typically switch all towns, but do not switch sidings between towns. The passenger car is parked in front of the station while the train switches cars in a town. Mixed trains have priority only over local freights.
When two trains of the same priority meet, for example two local freights, priority is usually decided by direction. You might set up your operations so that eastbound trains have priority, or so that westbound trains have priority. On the Lake Town & Shire, we have our largest yard with an interchange at Rivendell in the middle of the layout. This is where traffic becomes most congested. So our rule is that trains moving away from Rivendell toward either end of the layout have priority over trains traveling toward Rivendell.
Every train is either included in a timetable or it is not. Any train that is not in the timetable is called a special. Since they do not have a timetable, specials operate according to train orders issued by station masters. With operators as young as six years old, we do not try to use a timetable on the Lake Town & Shire. All of our trains are specials. Our train orders are verbal. A train does not leave any town until the station master gives it permission to proceed to the next town, switching any sidings along the way. We have three stretches of single track mainline on the Lake Town & Shire. If we do not have station masters and a train is about to go onto one of the single track sections of mainline, then the conductor must check to make sure there is no train about to leave the next town. If there is a conflict, the train traveling away from Rivendell have priority.
We do not run passenger trains, express freights, or mixed trains. Our operators enjoy switching cars. Since local freights provide the most opportunity to switch cars, we run only local freights on the Lake Town & Shire.
Maximum train length is determined by the number of trains you want to run, your layout design, and the length of your passing sidings. If you want to run only one train at a time, or if your layout has double-track, then in principle you can run trains as long as your motive power can pull. The same is true if you have a loop layout, all your trains move in the same direction, and no train ever passes another train. That works fine for passenger trains and express freight trains. For local freight trains, though, you may want to keep your trains shorter. It is very difficult to back a long train through a turnout without a derailment. Of course, if two trains ever meet, or if one train ever passes another, then you will want to make sure that at least one of the two trains will fit on the available passing siding. There are complicated movements that will allow two trains, both too long to fit on the passing siding, to pass each other, but you probably don't want to try that - especially with young operators. On the Lake Town & Shire, every passing siding will hold a locomotive and tender, a caboose, and three freight cars. So that is the maximum train length we allow for local freights. We could allow one express freight or one passenger train at a time on the mainline that was longer than this if we chose.
Every real passenger train has an engineer and a conductor. If it is pulled by a steam locomotive, then a real passenger train will also have a fireman. On a model railroad, you don't need the conductor or the fireman. So a passenger train on a garden railroad has a crew of one: the engineer. I don't have any more to say about crews for passenger trains.
Every real freight train has an engineer, a conductor, and at least one brakemen. If it is pulled by a steam locomotive, a real freight train will also have a fireman. On a model railroad, you don't need the fireman. So a freight train on a garden railroad has a crew of one, two, or three with the roles of engineer, conductor, and brakeman. On the Lake Town & Shire, we run crews of two: an engineer and a brakeman with the more experienced operator also serving as conductor. The conductor gives directions, the engineer operaties the locomotive, and the brakeman switches the turnouts and uncouples the cars.
When we have several relatively inexperienced operators attending an operating session, we sometimes assign two of these to a crew as engineer and then assign a third experienced operator as conductor. We also sometimes assign an third, inexperienced operator to a two-man crew as an observer.