SupeBloc Fundamentals

by John Matthews © 2005 Exeter N Gauge Railway Modellers

What is superBloc?

SuperBloc should prevent collisions between trains that follow one another along the same track. It is based on the Absolute Block System that was made mandatory on UK railways in 1889 and has been in continuous use ever since.

Unlike DCC, superBloc allows a single operator at home to keep a dozen trains all running at once on the same or conflicting routes. You have the feel of being in control of a railway. There is no limit to the size of a superBloc-controlled model.

UK Railways are Block Controlled
In the earliest days, trains were few and could travel between stations at timed intervals. Increasing traffic led to some spectacular accidents, especially when the newer fast passenger trains caught up slower trains, or ran into breakdowns. The solution was to divide the railway into block sections each of which was allowed to contain only one train at a time. Each block section was put under the control of a signalman and drivers were obliged to obey their signals.

Signalmen in boxes are in contact with one another, originally via block instruments linked by telegraphs. Before Signal box A can pull off his starter signal to allow a train to proceed, he must first ask signal box B whether his block section is clear as far as signal box B’s Home Signal. Similarly, signal box B must get permission from his block ahead to pass the train forward. If not, his Home Signal remains at danger and the train from signal box A must stop. In case the Home signal isn’t visible, a Distant Signal that copies the Home Signal tells the oncoming driver where to apply his brakes.
If a train is to stop in the station, and the platform is free, it is brought almost to a stop at the Home Signal. The Home signal is then released to allow the train to pull forwards slowly into the platform. The Starter Signal can be released as soon as the Block Ahead is proved to be free.

A Signal Passed At Danger (SPAD) still makes regular news today. In practice a short overlap is included after the Home Signal to prevent a collision if the driver does not quite stop. Drivers who SPAD are nevertheless reprimanded and risk the sack. Being a driver is not the fun it is made out to be – you must do as the signals tell you! As a signalman, however, you are in effective control of all the trains that pass through your section.
Eventually, the tracks between stations were divided into multiple intermediate block sections (IBs) each long enough to allow the fastest train to stop. Each new block has a set of signals of its own. A train can then leave SB A while SB B’s block is still occupied, provided that one IB ahead is free. Adding IBs increases traffic density. Today a single power signal box is connected electrically or electronically to all the blocks in a large area so that relatively few signalmen can control a large number of trains.
This is the ideal prototype for a lone operator at home wanting to run a large number of trains simultaneously on the same or conflicting routes, or for a relatively few operators to run a high traffic density on a large complex exhibition layout.

Enter superBloc
SuperBloc emulates all the features of prototypical block control by driving locomotives automatically to obey the signals. Thus operators experience the signalman’s sense of being in control of all the traffic on a complete railway.
SuperBloc employs a special Block Controller. Its method of operation was invented in 1998 and has subsequently been refined seven times as a result of experience at exhibitions. The current controller, the BC3 revision 4, now revision 5, has proved to be effective and robust. Yet the principle of block control is so powerful that all superBloc controllers from the very first prototype to this latest version can be linked together on the same layout. Having operated the real railway (the prototype, as modellers call it) since 1889, block control is inherently future-proof. Only the technology has changed.

How superBloc controls trains.

Each route is divided into block sections long enough to hold the longest train that will be run (see Figure 2). Block Sections are separated by double track breaks. Each block is then wired to a BC3 controller.
All BC3s are fed from the same Power Supply Unit (PSU) of nominally 20 Volts DC rating. Two wires then link BC3s together between adjacent blocks.
The first wire (called Busy) tells the block in the rear when the block ahead is occupied. If it is, any train arriving in the block in the rear is slowed to a halt by that block’s BC3 and released only when Busy ahead indicates clear again. If the block ahead is clear when the train arrives, the train proceeds at its target speed. The BC3 employs normal DC inertia controller methods to slow and stop the train, then to accelerate it again back to the target speed. It is therefore suitable for all types of DC locomotive.
The second wire passes the speed setting forwards from the block in the rear to ensure speed continuity between blocks.
Stopping at a signal
The BC3 knows when it is busy by the current that it supplies to a locomotive. Current sensing provides the track circuit function that generates Busy. Experience shows that locomotives respond quite differently to inertia control. The BC3 therefore implements a strategy for stopping any train close to its block’s Home Signal. Figure 3 shows how it works.
As a train enters a block where must stop, its speed is first ramped down by the inertia controller until it is just creeping along at the slowest possible speed. Just before the Home Signal, a sensor in the track detects the train and causes the BC3 to reduce speed again, this time until the train stops at the signal. The two ramping rates can be separately adjusted. The signal is placed at an overlap distance before the end of the block to accommodate minor “SPADs”, as in the prototype.
The sensor can be as simple as a track break with a diode across it, although the BC3 caters for all the common forms of optical and magnetic sensor. In diagrams that follow, a sensor track break is shown.
Automatic Distant, Home and Starter signals can be operated from Busy using signal drivers described in superBloc documentation.

An operator’s control panel typically takes the form of a signal box mimic diagram of the tracks with knobs, switches and LEDs. The BC3 provides for an occupied LED to indicate the presence of a locomotive. The signalman-operator can also have a Home Signal lever (ie a switch) to hold his Home Signal at Danger, over-riding a ‘clear’ indication from ahead. In addition he can have a lever (or switch) to force a Busy indication to the block in the rear thus preventing acceptance of a train into his block even if it is clear. This, for example, allows a derailment to be replaced without affecting any other block.
Blocks are independent of one another
Each BC3’s track connection is current limited. This both protects a locomotive from burnout should it stall, and prevents the power supply from tripping out on a short circuit. Thus an accident that causes a short still allows traffic movements to continue everywhere else. Any problem can therefore be dealt with locally while the rest of the layout continues to run. The PSU trips out only if there is a wiring fault or you need a larger supply because you are running too many trains!

Manual control for shunting
Each BC3 has optional manual inputs to allow shunting within the block it controls. Figure 4 shows the connections.
When Shunt is selected, the operator can drive the train anywhere in the relevant block, independently of what is going on in adjacent blocks, using conventional speed and reversing controls. A Busy indication is sent to the block in the rear whether or not a loco occupies the block. This, for example, enables a loco to be parked in an isolated section of track just as in conventional DC or CAB control without the block appearing to become clear.
When in Shunt mode, the BC3 behaves just like a conventional DC controller as used in everything from Toy Train Sets to Progressive CAB control. SuperBloc is therefore compatible with these earlier control methods and can be implemented as a smooth upgrade from them.
For convenience in wiring, the connections to a BC3 controller are made via three 5-pin plug-in connectors that are standard between BC3 versions. Connector 1 links to the track and PSU, Connector 2 to the Manual control panel, while Connector 3 carries the wires that handle block interactions.
SuperBloc documentation shows how adjacent blocks can be linked to allow shunting between them. As a result, a loco can, in principle, be shunted around the whole layout, but this requirement would not normally occur.

Single Line working
SuperBloc documentation also shows how to wire sensors at both ends of a block and to connect the BC3’s reversing input to implement single line working. It is sufficient here to say that superBloc provides for the control of a single line using electronic tokens to regulate traffic.
What happens at junctions?
There are two basic junction types, a converging junction, eg where a branch line or loop joins a main line, and a diverging junction, eg where a branch or loop begins. Once the rules for connecting these are understood, they can be applied to any kind of junction, however complex.

The Converging Junction
A converging (or trailing turnout) junction (or the first converging junction in a complex junction sequence) must always be where two blocks meet. Why? If two trains are approaching the junction simultaneously, the one with the turnout (or turnout) set against it must stop at the signal. Since a train can approach on either track it follows that both tracks must be able to hold a stopped train, and must therefore be long enough to form blocks. More on this later…
The train approaching the junction with the turnout set for it has to stop only if the block ahead is occupied, or the signalman sets the signal at danger, eg to give a train waiting on the other track priority.
The switching shown fully implements these rules. Both approach blocks’ Stop inputs are connected to the Busy of the block ahead via diodes that prevent the stop input to either approach block from affecting the other. The switch connects the Stop for the block whose train cannot proceed because the turnout is set against it to the PSU’s 0V, which guarantees a stop.
A second 2-way switch transfers speed between appropriate blocks.
By linking the switches to the turnout, eg using a Peco PL-15 auxiliary twin micro-switch, the whole operation becomes automatic simply by changing the turnout. The signals can be operated automatically from the two Stop inputs to give their correct indications.
SuperBloc documentation gives alternative circuits using relays to operate both the above connections and the turnouts themselves with greater reliability.
The diverging Junction

A diverging (or facing turnout) junction (or the first facing junction in a complex sequence) is wired up as in Figure 6.
This time, only one switch is required to select which block ahead will supply the Busy input to the approach block. A Peco PL-13 turnout auxiliary switch (or the more reliable relay equivalent) makes the process fully automatic.
Speed can be passed forward to both blocks because the train will enter only one of them.
Signalmen on the prototype are required to set signals to danger before changing the turnouts – interlocks between the various operations prevent them from doing otherwise. In the above, changing a turnout sets the signals. The operator should therefore take care not to change a turnout or set a signal to danger in a block that is already occupied unless the train has already stopped. Interlock wiring can be provided to enforce this rule but, in the model situation, it may be considered unnecessary or even too restrictive.
What constitutes a Block?
Figure 7 shows two versions of a crossover between parallel tracks in the same direction.
The upper example shows a simple crossover where both turnouts are operated simultaneously by the same lever. If the crossover is normal, a train from Block A proceeds to Block C while a train from Block B proceeds to Block D. Before the crossover can be reversed to allow a train from Block B to enter Block C, either a train entering Block A must be stopped or it must be allowed to clear Block C. A following train intended for Block D will be held up.
The switching required is simply that for a facing junction connected directly to that for a trailing junction.
A block section must therefore be long enough to stop and hold the longest train that will pass over it. When designing layouts, thought is given to the placing of turnouts to allow varied routes. When implementing Block Control, thought has to be given to the effect that train interactions might have in restricting traffic movements.
Comparison with conventional control and CAB Control
Most people begin by wanting to drive a locomotive. They buy a Toy Train Set. This allows a single train to be controlled on a layout that can easily be extended. When a second locomotive comes along, the two trains can be run simultaneously, provided that they occupy non-conflicting routes.
As soon as more locomotives are added, or trains must share routes, the conventional solution is CAB control, which divides the layout into sections that can be switched to the appropriate controller ahead of each train. Each controller requires an operator, while another operator must work the switches. Very quickly, the scenario becomes a nightmare.
One solution is to control the switches by a computer (leading to Progressive CAB Control – PCC). This involves adding feedback from track circuits so that the computer knows where the trains are and can set signals appropriately. Either you need the knowledge of how to do this or you pay a lot of money for the computer interfaces and software. As the layout grows in complexity its problems escalate.
SuperBloc provides an alternative way to run many trains on a complex layout. The trains can all be kept running on conflicting routes by a relatively few operators acting as signalman. The layout is infinitely expandable – however large it becomes, the rules for interconnecting and interlocking blocks together remain the same. This is why the prototype, though large, is so readily controllable.
For those who want it, a computer can be connected to provide a supervisory layer but, if the computer is not present, superBloc continues to prevent trains from colliding with one another. Operation does not depend on a computer.
Comparison with DCC
DCC was introduced to give those who want to drive a locomotive a more satisfying experience. It places a controller (called a chip) in each locomotive and operates it by digital codes transmitted through the track from a Command Station. Each locomotive is programmed with its own speed and acceleration characteristics, which can be matched with other locomotives. Lights, bells, whistles, and smoke can also be controlled.
One benefit that attracts many to DCC is that wiring is simple. Only two wires are needed between the command station and the track. Several locomotives can be run on the same track at the same time and there is no restriction as to the number of locomotives in the same section or block. This makes it ideal for large American layouts where a long train may include several locomotives, for shunting yards, and for engine sheds.
In addition, turnouts and signals can be operated from the command station through the track.
However, it isn’t practical to run a chipped locomotive on a DC (or superBloc) layout. Nor is it practical to run an unchipped locomotive on a DCC layout (though documentation suggests that you can). As a result, once you have adopted DCC you are locked in.
The main snag with DCC is that it is designed specifically as a transmitting device. It does not receive feedback to indicate where locomotives are. Block control would therefore require an extra layer of control. Every locomotive, turnout, and signal needs an operator.
Recent competition from block controlled systems, including superBloc, have left some DCC users dissatisfied. Some manufacturers have tried to add feedback to DCC to allow block control to be implemented. But since this was not contemplated when DCC was conceived it has led to a number of manufacturers departing from the DCC standard.
The received opinion at present is that to achieve Block Control with DCC you need to add a separate computer-control layer to process track circuit information, and to work turnouts and signals, with complex wiring. And you still need operators to drive your trains. It is impractical for a single operator to drive lots of trains simultaneously without yet another computer interface and software, but this leads to added complexity and spiraling costs.
Compare this with superBloc, where Block Control with automatic driving is implemented at a fundamental level to emulate the prototypical approach.

The benefit of the superBloc approach is that it is progressive. You begin with a Toy Train set and build up. Costs grow with the size of the layout – there is no big expenditure up front for a Command Station. And yet, if you wish, you can finish up with an automatic computer-controlled layout.
The biggest advantage of superBloc is that, even at an elementary level, a single operator at home can be in effective control of a layout running a dozen trains at a time on conflicting routes.
Where can superBloc be obtained?
SuperBloc control is suitable for all gauges and has been demonstrated on 00, N and Z gauges.
SuperBloc documentation is available free to members of the Model Electronic Railway Group (MERG) by download from its website. Kits of parts for building BC3 controllers and components to implement interlocking are also available to members at cost.
A taster document, MERG Technical Bulletin T33/01, can be downloaded in PDF format from the public pages of the MERG website