Except for the very smallest railroads which may be operated under yard rules with only one train at a time, inter-city railroad lines are controlled by a dispatching center. Dispatchers track the movement of trains and grant permission to occupy segments of track. They also grant permission to various maintenance and inspection vehicles to use track segments without interference from trains.
Railroads generally issue authorities to operate based on six cascading systems:
Federal laws (and some state laws) regulate train operations. Limits to work hours is one example of where the law is controlling. During the late 1800's through the 1970's, Michigan passed numerous laws regulating railroads, the most common being limits for blocking highway crossings. However, revisions to federal law and various court decisions have made most of these state laws and local ordinances unenforceable.
Railroads adopt a "book" of operating rules, the most common in Michigan being the General Code of Operating Rules. Michigan railroads which have adopted the General Code are CP, Watco group, Rail America group, LS&I, A&B group and Amtrak Michigan lines. In the old days, railroads wrote their own operating rules but this concept has generally changed to adoption of a national standard for rules. The other prominent book of operating rules are the NORAC rules (Northeast Operating Rules Advisory Committee). Railroads adopting NORAC are Amtrak (NE corridor), Conrail, CSX and NS. Operating employees must review rules and they are tested about them before they can be "qualified" to do their job on a particular route.
Timetables include a listing of routes, stations, train schedules and special instructions. Special instructions might include speeds, speed restrictions, track clearances, etc. In the heyday of railroading, employee timetables were usually issued every six months but sometimes more often. Many railroad historians collect old employee timetables, which go back as far at the 1860's and they are an excellent resource to examine the old railroad network in Michigan. Railroads also produced public timetables for passenger trains and even some freight schedules for public view. These often had attractive covers and are also coveted by collectors. But these public time tables were not binding on railroad operations - they were simply information for the public to use in scheduling transportation.
At any time, railroads issue bulletins to modify timetables and even operating rules. This might happen to modify the schedule of a train between regular issuance of a new timetable. Other bulletins include speed restrictions, track in and out of service, warnings about things which are discovered, etc. Before a train can proceed, the dispatcher must verify that a crew is qualified and has the latest copy of daily bulletins.
Track Warrants and Mandatory Directives (formally Train Orders)
Track warrants are used by many railroads which give permission for trains to operate various track segments along with specific instructions on where to meet other trains, speed restrictions, train following, unusual crossing protection, etc. The use of Track Warrants are granted in the GCOR. Track warrants can modify bulletins, timetables and in some cases Operating Rules. Some rail lines are controlled using Mandatory Directives. As an example, a train might be proceeding from Detroit to Lansing under signal indication in a CTC area when the dispatcher is informed about non-working signals at a highway crossing. The dispatcher will call the train by radio and issue them a mandatory directive to stop and flag that particular crossing. For subsequent trains, the stop and flag order might be added to the track warrant.
Radio instructions might modify track warrants and mandatory directives, but this is usually a formal process of modification in which radio is simply the communications medium to make the modification. These orders are repeated and verified. Most railroads record their radio traffic to verify the documentation. Railroad radio began in the 1950's and was not routine in train engines and cabooses until the 1970's. Prior to that time, railroads used signals and train order signals to stop trains and summon the Conductor and Engineer to wayside telephone booths. Now, in almost all cases, each train crew member has a radio and most crews keep the Conductor and Engineer in the engine cab. Cabooses are only used for railroad switching functions.