Moving Block vs. Fixed Block: The Battle for Railway Capacity Explained

At 8:30 on a weekday morning, the Jubilee line carries approximately 32,000 passengers per hour through the 3.1 km tunnel between Westminster and London Bridge — a density of roughly 10,000 passengers per tunnel-kilometre. Before its CBTC moving block upgrade in 2011, the same infrastructure carried approximately 24,000 passengers per hour. The infrastructure did not change. No new tracks were built, no stations added, no tunnels widened. The capacity increase came entirely from the signalling system: moving block, by shrinking the separation distance between trains in proportion to their speed, allowed trains to run every 100 seconds instead of every 150 seconds. Eight trains per hour extra, through the same tunnels, serving the same stations, on the same tracks.

This is the capacity story of moving block: not bigger infrastructure, but smarter use of existing infrastructure. Understanding it requires understanding precisely why fixed block systems create capacity ceilings — and why moving block removes them.

Fixed Block: The 19th-Century Logic That Still Governs Most Railways

A fixed block is a defined section of track, bounded at each end by a signal. The rule is absolute: only one train may occupy a block at a time. When a train enters a block, the track circuit (or axle counter) detects it and the signal at the block entrance shows red. No following train may enter until the preceding train has cleared the entire block and the next signal has cleared to allow entry.

The safety logic is robust. A following train cannot be authorised into a block that the leading train occupies, regardless of where in the block the leading train actually is. If the leading train is at the far end of a 1,500-metre block, approaching the exit signal, the following train still cannot enter from the entrance signal until the block is fully cleared.

The capacity problem is geometric. At any given moment, the minimum separation between the back of the leading train and the front of the following train is:

Minimum separation = (length of occupied block) + (braking distance of following train at approach speed) + (train length)

On a 3-aspect system at 80 km/h: 1,500 m (block) + 800 m (braking) + 120 m (train) = 2,420 m minimum nose-to-tail separation

Even if the leading train is stationary at the far end of the block, the following train must still clear two full block lengths before the first signal clears to green — because a 3-aspect signal system provides one yellow “caution” block between the red-occupied block and the green-clear signal. The result: at 80 km/h, a minimum headway of approximately 2.5–3 minutes.

Moving Block: The Dynamic Safety Zone

Moving block replaces fixed geographic block boundaries with a continuously calculated separation. The fundamental measure is no longer “which block section is occupied” but “what is the minimum distance required for the following train to stop before reaching the rear of the leading train, given both trains’ current speeds and the leading train’s deceleration characteristics?”

This calculation — performed multiple times per second by the train control system — produces a dynamic movement authority for the following train: “you may proceed up to position X.” As the leading train moves forward, X moves forward. As the leading train brakes, X slows its forward movement and eventually stops advancing. As the following train approaches the calculated stopping point, it receives a continuously updated target distance and target speed, enabling smooth braking rather than the stepped “green-yellow-red” approach of fixed block.

The Braking Curve: How Target Distance Braking Works

In a fixed block system, a train driver (or ATP system) reacts to signals in a step-wise way: green means full speed, yellow means prepare to stop at the next signal, red means stop. The braking is reactive and conservative — the driver must be able to stop within sight of the yellow signal, which means approaching the area of caution at reduced speed before the actual obstacle is even reached.

In a moving block system, the following train knows the exact position and speed of the train ahead, and its movement authority includes a precise target point (typically the rear of the preceding train plus a safety margin). The ATP system calculates a smooth, optimal braking curve from the current position and speed to the target point — braking begins later than in fixed block, and the stopping point is precisely the movement authority limit rather than a conservative approximation of it. This “target distance braking” is both more efficient (less unnecessary braking) and more precise (the train stops exactly where authorised, not one or two block lengths short).

The Capacity Numbers: Fixed Block vs Moving Block

Full Comparison: Fixed Block vs Moving Block

CBTC: Moving Block in Practice on Metro Systems

Communications-Based Train Control (CBTC) is the dominant implementation of moving block on urban metro and light rail systems worldwide. A CBTC system consists of:

• Onboard positioning: Tachometers, inertial measurement, and wayside transponders (balises at fixed intervals) provide continuous precise train position to ±1 metre accuracy.
• Continuous radio communication: Each train transmits its position, speed, and status to the zone controller multiple times per second, and receives movement authorities in return.
• Zone controller: The central computer calculates movement authorities for all trains in its zone — essentially running a continuous real-time model of all train positions and braking curves.
• Interlocking integration: CBTC interfaces with the CBI to verify point positions and route clearance before issuing movement authorities that traverse junctions.

CBTC systems are in revenue service on over 200 metro lines worldwide, including virtually all major new metro openings since 2000. Lines that were originally built with fixed block signalling and have been upgraded to CBTC — including the London Jubilee line, New York City Subway’s L train, and Singapore’s North-South and East-West lines — have achieved documented capacity increases of 20–35% without any track construction.

ETCS Levels and Moving Block: A Precise Distinction

A common misconception is that ETCS Level 2, which uses radio to communicate movement authorities, is a moving block system. It is not. ETCS Level 2 authorities are still calculated on the basis of fixed track sections — the RBC knows which track circuits are occupied and issues movement authorities to the edge of the last clear section. The radio communication is a more efficient and flexible way of delivering fixed-block authorities to the cab, but the underlying separation logic remains fixed-block. True moving block requires train position reporting — the train telling the system where it is, rather than track circuits telling the system which section is occupied.

The ETCS Level 3 Barrier: Why True Moving Block Has Not Reached Mainlines

ETCS Level 3 — the specification for true moving block on mainlines — has been formally defined for decades but has not entered mainline revenue service anywhere in the world as of 2026. The fundamental barrier is broken rail detection.

Track circuits detect broken rails: a fractured rail opens the electrical circuit and the signalling system shows the section as occupied, stopping trains before they reach the break. In a moving block system that has eliminated track circuits, there is no infrastructure-based mechanism to detect a rail fracture. The train reporting system cannot detect a broken rail — it only reports the position of trains that are operating. A broken rail in an empty section would be completely invisible to the system until a train was close enough to detect it from the cab — at which point it may be too late to stop.

Solutions being researched include DAS (Distributed Acoustic Sensing) via fibre optic cables for continuous broken rail detection, rail integrity monitoring through acoustic emission sensors, and statistical analysis of train vibration data. None has yet achieved the combination of detection reliability and safety certification level required to substitute for track circuits in a mainline SIL 4 safety case. Until this problem is solved, ETCS Level 3 moving block on busy mainlines where broken rail is a realistic risk remains a specification without a deployment.

Editor’s Analysis

Frequently Asked Questions

Q: If moving block reduces separation, does it make railways less safe?

No — moving block maintains equivalent safety to fixed block while reducing the conservative spacing that fixed block requires beyond the actual braking distance. In fixed block, the minimum separation is the length of one or more full block sections, which may be 1,000–2,000 metres even at low speed. In moving block, the minimum separation is the actual braking distance — which at low speed approaching a station may be only 100–300 metres. The safety case for moving block is based on the precision of the position-reporting and braking-curve calculation: if the following train always knows the exact position of the train ahead and always maintains a separation equal to or greater than its own braking distance, the probability of collision is equivalent to or better than fixed block. The higher precision of moving block position knowledge (±1 metre versus “somewhere in a 1,500-metre block”) is actually a safety improvement in some respects — the system knows exactly where each train is, rather than only which block section it occupies.

Q: What is the difference between headway and frequency?

Headway is the time interval between successive trains at any given point on the line — the gap between one train passing a fixed point and the next train passing the same point. Frequency is expressed as trains per hour: a 90-second headway equals 40 trains per hour; a 3-minute headway equals 20 trains per hour. The two terms are mathematically related (frequency = 3,600 / headway in seconds) and are often used interchangeably. In capacity analysis, headway is more useful because it directly relates to the signalling system’s capability; frequency is more useful in timetabling and passenger communications. The capacity of a line in passengers per hour is the product of frequency (trains per hour), train capacity (passengers per train), and the load factor at which the operator is willing to run.

Q: Can a fixed block and moving block train run on the same track?

This mixed-fleet scenario is one of the most challenging aspects of moving block deployment on existing railways. On a CBTC metro system, the standard approach is to equip the entire fleet before switching to moving block operation — the CBTC system may retain the old fixed block signalling as a fallback for any non-equipped vehicles. On a mainline ETCS system, non-equipped trains can operate in a fallback mode where they follow conventional lineside signals, while equipped trains receive radio-based movement authorities — but the capacity of the line is limited by the slower (fixed block) mode of the non-equipped trains, negating much of the moving block benefit. This is why fleet-wide ETCS equipment is a prerequisite for achieving the full benefit of moving block on a mixed mainline route.

Q: How does moving block handle a train stopping unexpectedly in a tunnel?

When a train stops unexpectedly in a moving block system, it continues to report its position to the zone controller. The controller immediately updates the movement authorities of following trains — their target point is now the rear of the stationary train at its reported position. Following trains receive updated authorities and begin braking toward the new target. The transition from normal operation to emergency stop authority is seamless and near-instantaneous — the following train’s authority shrinks in real time as the leading train decelerates, rather than waiting for a track circuit section to show occupied and a signal to change. In practice, a CBTC system’s response to an unexpected stop is faster than a fixed block system’s, because the authority reduction begins as soon as the leading train starts decelerating rather than only after it has fully occupied a new block section.

Q: What is a “virtual block” and how does it differ from true moving block?

A virtual block (also called a “variable block” or “pseudo moving block”) is an intermediate approach used in some ETCS Level 2 implementations: the track is divided into very short fixed sections (50–200 metres), so that the minimum separation approaches what a true moving block system would achieve, without eliminating track-based detection. Virtual blocking provides most of the capacity benefit of moving block while retaining track circuit-based broken rail detection. The disadvantage is the infrastructure cost of short-section track circuits — at 100-metre spacing, a 100 km route would require 1,000 track circuit sections and the associated insulated joints, cables, and receivers. True moving block eliminates this infrastructure cost but requires an alternative broken rail detection solution. Virtual block is often described as a practical interim step toward full moving block, usable on routes where the broken rail detection gap cannot yet be closed but higher capacity is needed.