Moving Block vs Fixed Block Signalling - Which is Better?

Traditional fixed block signalling grew out of the invention of the track circuit which gave the signalling community its first failsafe method of detecting the presence of a train. This method has served the signalling community well for the past 150 years but as major urban centres grow, the demands on transit operators also grow.

Fixed block signal engineers include a design margin which allowed the operator to increase the frequency of service, but many cities have exhausted this margin or are fast approaching it.

As a result, transit operators have two choices; build more subway lines or squeeze more out of their existing infrastructure by adapting new technologies; such as moving block based on CBTC technologies.

Moving block signalling is a concept that grew out of the CBTC world because it removed the need for track circuits as the primary mechanism for detecting the presence of a train. Since track circuits forced signal engineers into a fixed block mindset, removing track circuits liberated designers into a brave new world of moving block.    

Fixed Block Signalling

If there is one sentence to describe fixed block signalling, it should be:

Fixed block signaling creates an artificial separation between trains.

Let me explain.

The primary purpose of a signalling solution is to keep trains safely separated by providing enough distance to stop the train in case of failure. However, train separation cannot be too long otherwise throughput is affected; instead of a train arriving every 3 minutes, trains may arrive every 5 minutes (headway).

Fixed block signalling divides the track into small blocks which determines how far apart trains will be kept for safety and how frequently stations will be serviced. The challenge for signal engineers is to size these blocks for optimum headway and safety at the same time; adjust the block for safety and headway is affected, adjust the block for headway and safety is affected.   

This is best understood through a simple example.

If train 8 hits a trip stop travelling at 60kph and it requires 200m to stop, the block separating train 8 from train 9 must be at least 200m long to satisfy the safety requirement.

As train 9 travels away from signal C, train 8 will not be granted a permissive aspect until block 3 is unoccupied. Instead, an artificial separation is created between train 8 and 9; even though train 8 can move closer and still maintain a safe braking distance.

The result is an unnecessarily long headway.

If the length of the block is increased, then the safety margin increases at the expense of longer headways. Shortening the block shortens headways at the expense of safety. This is the nature of fixed block signalling and several problems emerge:

1.   A block may be 200m long but if the train is only 100m long then the space within the block is wasted (see the example above). The signaling system considers the length of the train to be the same as the length of the block - if the block is 200m long, the train is considered to be 200m long.

2.   If the calculated block size is 200m based on a train travelling at 60kph, then a train travelling at 40kph would also be forced to maintain a 200m separation.

Unnecessarily long headways handcuff the operator in terms of the number of trains they can push through the system because a fixed block design does not utilize the track in an efficient manner.

Fixed block signalling is a solution based on a technology invented in the 1860’s. Today with the power of microprocessor based systems, it is time to look to new and more efficient methods of controlling trains.

Moving Block

With the advent of CBTC, Signal Engineers started to view the guideway as one contiguous track rather than a chain of small blocks.

This was a radical departure from the traditional view of signalling. Moving block upended fixed block signalling concepts that had developed over the past 150 years.

The most important feature to pop out of this new paradigm is the safety distance was no longer a static entity enforced by fixed blocks, but an adjustable distance based on a real time calculation of the train speed; if the train is travelling at a high speed the safety distance is long and shrinks if the train is traveling at a slow speed.

The immediate benefit of this approach is the headway between trains is reduced.

Another aspect of moving block is the safety distance moves with the train, hence the name moving block. It’s no longer a static entity defined by the location of signals and trip stops.

As the leading train moves forward, the preceding train follows while maintaining a safety distance separation. Since the separation is kept to a bare minimum, there is no wasted space, the train is not left waiting for a block to clear (as it is in fixed block) and most important, the headway is kept as short as possible.

Moving block utilizes the track in the most efficient manner while ensuring safety.

Fixed block signalling was a first great step in the evolution of signalling, but it has out lived its usefulness at a time when all transit authorities are demanding shorter headways to move more riders using the same infrastructure.

But old ways die hard especially ones that have developed over the past 150 years.

Are you reviewing a CBTC Supplier's design & are overwhelmed by its complexity? Are you deploying a CBTC Solution and are facing field integration challenges? Are you defining your transit property's CBTC specification and are unclear which function to give priority to? If so, schedule a free 1 hour consultation. Sometimes a quick discussion can solve the trickiest problem.

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