One of the main reasons the superiority of trains is set up the way it is, is it allows operation when there is poor communication. The system was originally developed when the highest communication technology was the telegraph. Communication could fail due to heavy ice (breaking wires), high winds (breaking wires or wrapping them around each other), fires (burning poles), lightning strikes or thieves stealing the wire. As communication reliability improved the necessity to have a system designed to accommodate failure decreased. By the 1960’s engines had radios on them and there was the possibility to communicate directly with the crews. That enabled the development of various track authority systems that used direct communication. These systems for the most part did away with the superiority of trains and train orders.
TWC/OCS/Form D
The most common system I will call “Track Warrant Control” or TWC. The Canadian roads have a similar system and the NORAC Form D system is very similar also. While they are described as a successor to train orders, they are in many ways a completely different animal. They actually operate closer to a verbal CTC than train orders.
The heart of all the variations is a form that the dispatcher dictates and the crew fills out to authorize a train to operate in a certain portion of track. The form is structured and the crew basically fills in the blanks. There are specific “lines” for different options. The dispatcher tells the crew which lines to fill out and what to enter in the blanks. The crew then has to repeat that back to the dispatcher.
Unlike train orders the authority is issued incrementally, much like CTC works. A train is told to either hold the main track or clear the main track at the last named point. The points can be a station, a siding, mile post or identifiable point. If the trains have cabooses or if there are block signals the rules may allow multiple trains in the same direction in the same limits. If not the rules generall allow only one train in s same direction in the same limits (unless they limits are given as “joint” limits with another train). My advice to people is to pretend there is a block signal system and let the dispatcher given overlapping limits to trains in the same direction. It really adds to the flexibility.
Like CTC the advantage is flexibility. The dispatcher has a very short planning window and can change his mind fairly easily. Authority is issued in small easy to understand bites. The down side is it requires a communication system for the trains to talk to the dispatcher and if the authority is given in small bites, it takes a LOT of authorities to get across the railroad, resulting in a LOT of radio traffic.
TWC rules have also evolved over the years and various aspects have changed and changed back. The “after arrival of..” line has fallen out of favor and been reinstated. Following moves have been allowed and then eliminated and reinstated. Various railroads have revised the track warrant form, changing the order and numbering of the lines, ironically influenced by train order era dispatcher procedures.
On some model railroads they have skipped using the forms and just issue the authority verbally, the dispatcher using the form as a scrip, but the crew not having to copy it down. Verbal authority (sometimes having been called “mother may I”), is similar to this just generally not as scripted. Using the track warrant as a script is good because it standardizes the instructions and reduces the chances for error, plus it give a more “railroady” feel to the communication. Not repeating back the instructions cuts the radio chatter by half.
DTC
Direct Traffic Control or DTC is a system that breaks the railroad up into fixed blocks and then authority is assigned by blocks. It has a very similar feel to operating an older DC, toggle or rotary block switch layout. The railroad is broken into blocks, each block is named and the ends are marked with signs. A dispatcher gives a train a number of blocks and lists their names. In TWC a dispatcher would name points, something like “proceed from Anna to Dora on the main track.” In DTC the dispatcher would give the train blocks, such as “you have authority in 3 blocks, Anna, Bess, Cloy”. The blocks need not be named for the sidings.
This can be a very simple system and requires less radio traffic. If you are operating without cabooses and no block signal system, a following move in the same block would be prohibited. That limits how many trains you can cram into a territory. On a real railroads that’s a bigger deal where you can fit 10 trains in a block, its not as much a deal on a model railroad where typically a block is only one train long.
On both of these systems there are messages that are given to the train crews in lieu of train orders to cover non-movement conditions, speed restrictions, wide loads, gang protection, tracks out of service, etc. These messages are called “bulletins”. In a TWC railroad, all the trains get a “track warrant for bulletins” which is morally equivalent to a clearance in train orders. Other systems have similar message with a similar function (many of the basics are same as they were in the 1800’s).
The advantage to a modeler in both of these systems (and there may be other similar systems developed by individual railroads or vendors) is the flexibility and short planning horizon. The flexibility works well where the model dispatchers aren’t as skilled at planning and the train operation is highly variable.