Communication for Safety and Traffic Flow 
Clear and concise communications are critical on a real railroad. While no one will get killed by a model train as the result of a communications error, good communications will eliminate problems when trying to run multiple trains, just as it does on the prototype. Regardless, our goal is to simulate a real railroad; thus communication is a major role-playing activity.
An Order is a written superseding instruction from a superior (often in regards to the Timetable, but sometimes also the Rulebook).
~ Because its contents may override previously printed documents, an Order must be explicitly written out, and all affected employees must affirm that they have accurately received [EXPLANATION] the superseding instruction (separate copies are not required on a model railroad if all the affected layout users can access a single posted copy). Stations (e.g., any location, including freight/passenger depots and towers, manned by a Station Operator [EXPLANATION]) are sometimes equipped with a physical "Order Board" [EXPLANATION], which indicates to a passing train crew if there is a critical Order waiting for them that they must affirm with their signature.
All businesses have a Rulebook, sometimes euphemized as an "employee manual". It is the top-level source where standing instructions/policies (such as "employees must wash their hands before returning to work") are either posted directly, or have their ancillary manuals (such as Glossaries) specified. Railroads are businesses, and they especially require a Rulebook (it can be a digital book) as the authoritative place to specify safety and operating information. The Rulebook is the official mouthpiece for the railroads Superintendent, and it is updated whenever major permanent changes take effect. All employees must sign that they have received the latest Rulebook.
The model railroads Rulebook is the place where standing instructions for our simulated train crew (and we layout users) are kept. While most real railroads have standardized their rule numbering between railroads on a national level over the decades, this has mainly been to facilitate ease in re-training employees coming from a competitors railroad. It is more practical for 3rail modelers to impose their own rule numbering (as well as other communication schemes used on the model railroad). Every model railroad should have Rulebook, and at the very least it need only contain one sentence:
"Turn off the Main Power when you have finished using the layout.".
Between Rulebook printings, dynamic changes can be made using a superceding instruction (e.g., Order) called a Bulletin Order [EXPLANATION]. On a model railroad the Bulletin Orders should be kept in a location where layout users can review them before they start running trains.
*** Modelers who only operate trains in a non-opposing in-line manner can safely run their railroad using nothing more than the instructions written in the modelers Rulebook!
If you have ever ridden a bus or flown on a plane, you have been exposed to a slimmed-down Public Timetable. An Employee Timetable is where information about all of the days planned train movements are listed (as well as other details, such as unique information about a specific spur). The Employee Timetable is closely linked with the railroads standard clock [EXPLANATION], and displays snapshots of both When and Where a scheduled train is supposed to be. As such, a scheduled train must never leave ahead of its schedule. Timetables may be modified by an authorized authority, such as a Dispatcher [EXPLANATION], throughout the day (for example adding non-scheduled Extra trains into the days timetable), as long as all affected parties are informed of the changes before they are scheduled to occur.
*** Any modeler who may face an opposing or passing train situation needs to, for accident prevention, implement a Timetable!
The Timetable works in conjunction with the Rulebook, and the Rulebook must specify train Precedence. Precedence (a non-physical form of "staff and ticket" [EXPLANATION] control) determines which train has priority to use a segment of track, and is determined first by Class (e.g., which type of train has superiority over another), and then by Direction (e.g., which bearing the train is moving down a track).
Most model railroaders can employ a very simplified Timetable that looks more like a Public Timetable. They theoretically don't even need to tie the Timetable to a specific clock time, and can simply list the round-robin order in which the trains are to reach designated points.
A Warrant is a written permission slip from a superior, most commonly used for managing traffic flow in areas lacking controlled trackside signal lights.
~ In the past Warrants written out, but nowadays most (Track Usage Warrants in particular) can be transmitted directly via radio (the sender must still log that they gave permission however). A "flag" to tell a passenger train that there is a passenger to pickup is treated as a Warrant rather than an Order.
*** Warrants were devised only to make the running of a railroad more efficient (and thus able to handle additional traffic). A modeler who implements a Timetable can forego issuing Warrants, and still move all of their regular trains in a safe manner.
Signals are nothing more than a way for railroad employees to indirectly communicate in real-time. You yourself for example receive signals on roadways in the form of traffic lights and road construction signs. Railroad signals can come in many forms, such as for example a posted blue flag indicating "Men at Work" a locomotive whistle indicating a flagman should return, or a hand signal [EXPLANATION] given to an engineer by a conductor. The most commonly encountered signal is the trackside signal light (such as a green light indicating clear to proceed), which functions as an electrical Track Warrant (that doesn't have to be signed for). Signal types and meanings are railroad specific, and are thus described in that railroads Rulebook (in the same manner as each States unique roadway signals are described in the States DMV Rulebook).
The electronics in modern O-scale locomotives allows modelers to utilize signaling features that were not easily implemented in the past. You can ring the bell as a warning when backing, blow the whistle as an alert when crossing a road at grade, and most importantly, you can display meaningful timetable train marker lights [EXPLANATION] (a crucial safety feature, as all cars do not have to be coupled together to be a part of the same train).
Real railroads send their employees to class in order to learn the railroads unique signals. With our model we usually don't want to go to that extreme, so it is best that all signals used on the layout (and between operators) be kept SIMPLE and to a MINIMUM.
TIP: Even if you do not model a working trackside signal system, because prototype railroads will have them, you should at least place dummy trackside signal equipment on the layout for scenery purposes (and write in your rulebook that they are just scenery objects, and that employees should ignore their existance).
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