The typical train set that people start out with has a circle of track, perhaps one or two sidings, and one locomotive with coaches or wagons. Rather obviously, with only one locomotive, only one controller is required to operate the train set. (see figure one).
When adding a second train, one of two situations must apply. Either a siding or loop will be used to hold one train while the second is being run, or a second circle of track can be added allowing the two trains to run independently. In this second case a second controller must be added to supply power to the second train so as to allow both trains to run independently of one another.
Two completely independent tracks are not much better than one because if you cant cross trains from one track to the other then you can't do much more than with the simple train set. This is where crossovers between the two tracks come in (see figure two). Crossovers, as their name implies, allow a train to cross over from one track to the other. A crossover can be made using two points and is either facing if a train will pass across it forwards, or trailing if a train will need to be reversed to use it. In order to keep the two track circuits independent of one another, each crossover will need to be fully isolated in the middle. If it is not then the two controllers will interfere with one another when the crossover is set to pass a train from one track to the other.
This now leads us to another problem. If we isolate each circuit, one from the other, then what do we do when we want to run a train from one track onto the other. One controller can only run the train as far as the isolation gap. Once the train runs over the gap then it is no longer being controlled by the first controller and will come to a stop. That is, it will stop unless the second controller is also in operation. If so, then instead of coming to a stop as it reaches the gap, the train will momentarily speed up for the fraction of a second that it is being powered by both controllers (i.e.. while the pick-ups are on both sides of the gap. Then the train will slow to the speed set by the second controller. If the two controllers are set to different speeds then this will result in a noticeable increase or decrease in speed of the train as it crosses the gap. The momentary increase in speed as the train crosses the gap will probably not be very noticeable, but a change of speed there will be.
A way of minimizing this effect is to use two controller of exactly the same type. This will ensure that the two controllers behave similarly. The controllers can then be adjusted to the same setting before the train reaches the gap thus reducing the increase or decrease in speed to a less noticeable level.
This does nothing to eliminate the temporary increase in speed as the train crosses the gap and does not even totally eliminate the increase or decrease in speed as the train crosses the gap no matter how carefully you adjust the controller to try to get the speeds the same.
Surely there must be a way of eliminating this problem completely. In fact there is, and the answer is cab control. With cab control, a mechanism is provided whereby a train can cross from one circuit to another while remaining under the control of the one controller. This mechanism works the same way whether we have two circuits as in our current example, or two thousand circuits and two hundred controllers.
Stripped to its simplest level, the idea of cab control for each controller to have a switch for each track section. The controller is then either connected to a track section, or it is not. Thus in our example we can have controller one powering the outer circuit while controller two is powering the inner circuit (see figure three), controller one powering both circuits (see figure four), controller two powering both circuits (see figure five), or controller two powering the outer circuit while controller one powers the inner (see figure six).
This same basic principle can be applied to any two controllers and any two circuits on a model railway and therefore is extendible to include as many controllers and track sections as required.
That's all that there is to cab control. A very simple method of allowing each controller to access each track section by simply providing a switch for each controller for each section. This method also allows you to add extra controllers as required without affecting the existing circuits in any way, it also allows you to have dedicated controllers that can only operate part of the layout by omitting the switches for the rest of the layout from that controller.
There is only one problem with this design. What happens when you switch two controller to the same section at the same time? Well to start with, this indicates a desire to run two trains into the same section at the same time and on the real railways if this was done there would be an almighty crash when the two trains collided so we probably don't really want to do this anyway. Still there's nothing to stop us switching two controllers to the same section with this setup so we need to know what will happen when it does. Well, if you're using decent controllers with proper overload protection built in then what will happen is that your overload protection circuits will activate. Then all you need to do is switch off one of the controller's section switches, reset the overload protection circuits, and you're back in business. If you don't have adequate overload protection, or you don't like the idea of having to reset overloads if two controllers accidentally access the same section then you need to provide some form of interlocking to prevent two controllers from accessing the same section at the same time.
NOTE that whatever form of interlocking that you provide, it is essential that the person using each controller know whether his controller is connected to a particular section. There is not much point in having an interlock prevent a second controller from accessing a section if the person using it thinks that he has accessed it and innocently drives his train across the gap thus interfering with the other controller's operation anyway.
The simplest way to provide this interlocking when you have two controller is to use double throw centre off switches. These switches point to the left to connect the section to one controller, point straight up when the section is not connected to either controller, and points to the right to connect the track section to the second controller (see figure seven). This is an ideal setup for small layouts operated by one or two people where the control panel is placed between the controller and the direction of the switch indicates readily which of the two controller is in control of which section.
For a larger number of controllers, you have three options. You can have a central control panel operated by the train dispatcher who decides which controller has access to which track section, you can have a separate panel for each controller and use an interlock of some type to prevent a second controller from accessing an already selected section (and have indicator lights to indicate which controller actually has control), or you can use a system of local panels where each controller primarily has access to a particular group of track sections and the operator of that controller decides whether to give up control of one or more of "his" sections to another controller (again a visual indication may be needed to indicate that access has been granted).
Let us consider each of these three options in turn. Each option has various advantages and disadvantages and which is the best system for you depends on how you intend to operate your model railway.
The central control panel has one big advantage, simplicity. All you need for this method is a rotary switch having as many positions as you have controllers and an on/off switch (figure eight). The same circuit is repeated for each track section. All sections not in use will have their on/off switch in the off position. To allocate a track section to a controller simply set the rotary switch to the appropriate position, switch the on/off switch to on, and tell the person using the section that he has control. Once a section is finished with the train despatcher is told that the section is no longer required and he switches the on/off switch back to the off position. Operators get access to each section by asking the train despatcher if they can have a section. If the section is off then the despatcher can give it to them, otherwise he can tell them that it is in use by another operator. The only disadvantages to this method is that only one person has access to decide which controller controls what track sections and you will probably need a separate person to look after this. This method therefore works best with big layouts having lots of operators where a central co-ordinator is useful.
The second method, that of providing full interlocking between of number of separate panels, has the advantage of allowing each operator to control which sections that he has under his control without requiring him to ask someone else to give him control. The big disadvantage of this system is that the interlocking circuits required are rather involved and a lot of wiring is required for all but the simplest circuit. There are a large number of different ways of providing this type of interlocking so I wont go into any of the different ways that this can be achieved at this time. It is very important that circuits of this type be understood by the layout owner and hence if you wish to interlock a large number of separate panels each of which can control the whole layout, it is probably best that you work out your own method of interlocking.
The third method is a compromise between the first two. It gives each operator control over his own part of the layout without the need to request access from a central despatcher. When he wants to run his train onto a part of the layout under someone else's control he asks that person for access. The degree of visual notice of which sections are connected to your controller depends on the method of operation being used. One suggestion is to have everyone drive trains towards them. You set the road for the intending arrival, the operator controlling the departing sections where the train is currently located sets everything ready for the trains departure and switches the required sections to your controller. Any intermediate operators set the road for the train to pass through and set the required sections to you. Commencing with the operator at the current location of the train, each operator notifies the next in line when he has set everything up. Once the message reaches you, you can then safely drive the train towards its destination. Each operator can see when the train passes out of his control and can reset his sections as required. If this method is followed then no further interlocking is required. It also has the advantage of allowing the person driving the train to see the train at the time that it reaches its destination and needs to be stopped. The biggest disadvantage of this system of distributed local panels is that only those sections of the layout controlled by panels having operators can be used so if there are insufficient operators to man all panels only part of the layout can be operated. This problem can be partly overcome by duplicating local panels at remote locations so that the sections can still be operated when the local panel is not manned. In this case it is essential to remember not to touch the remote panel when the local panel that it duplicates is manned.
The circuits required for the third method are in fact identical to those required by the first method. The only difference is that not all of the switches will be located in the same place. In the case of a duplicate remote panel, the circuits are again the same. The important thing to remember with local and remote panels is that when the local panel is manned all on/off switches on the remote panel must be off and similarly the other way around otherwise the same problem will potentially occur as with the simple circuit originally described where different controllers can be switched to the same section of track.
There are many different ways of arranging control circuits so that a train can be run throughout its journey using just one controller, most of these involve the use of simple circuits such as those described in this article. If you have a large layout then you might need lots of these simple circuits but the difference between a small layout and a large layout is in the number of track circuits required, NOT the type of circuits that need to be provided.
Cab control enables you to run your trains in a more railway-like manner. The circuits do not have to be complicated (although they can be if you really want to provide lots of interlocks). Cab control also has one big advantage over the one controller / one section of the train set. If you have a number of areas where it is possible to run trains independently then with one train per section each of these areas will need their own controller. With cab control, you only need as many controllers as the number of trains that you intend to run simultaneously. Further controllers can be added as and when required.