Deltron Equipment & Systems Pvt. Ltd.
Intermediate Block Signalling using MSDAC & UFSBI
Introduction to Intermediate Block Signalling Concept
​
Intermediate Block (IB) signalling is a strategic solution employed by Indian Railways to significantly enhance the utilisation of existing infrastructure on high-density double-line or in some rare cases single-line routes. It involves the division of a long block section between two stations into a pair of smaller subsections, known as Intermediate Block Sections, each governed by its own dedicated signaling arrangement. This enables the simultaneous occupation of the original block section by two trains—one in the rear portion and another in the forward portion (generally termed as Block section) —under strict interlocking controls. By enabling more than one train movement within what was previously a single occupancy section, this scheme can increase line capacity by approximately 20–25% without the need to construct an additional railway track. The result is improved operational flexibility, reduced headway, and enhanced punctuality, making it an effective measure for addressing traffic saturation in key corridors.
A working Intermediate Block Signalling Scheme using UFSBI & MSDAC on Dual Media on Double Line Section
Vital Input /Output Exchange Using UFSBI
Overview of UFSBI
The Universal Fail-Safe Block Interface (UFSBI), is most widely used critical safety subsystem approved by RDSO for application within Indian Railways. Deployed in mostly dual redundant configuration for this application , UFSBI facilitates the reliable and secure exchange of safety-critical input and output signals between station interlocking systems and Intermediate Block Huts (IBHs). Its architecture ensures a fail-safe communication environment essential for the control and supervision of signals and related functions.
​
Operational Characteristics of UFSBI in IB Signalling
UFSBI enables the exchange of vital signalling input and output states for both UP and DOWN lines, employing a robust fail-safe communication protocol. It is capable of operating over dual media, including configurations of dual Optical Fibre Cable (OFC), or hybrid arrangements involving quad cable, dark fibre, or E1 (2 Mbps) channels with automatic media changeover arrangement. The system ensures the integrity of the interlocking by securely transmitting vital commands and indications. These include the aspects of the Intermediate Block Signal, IB Distant Signal, and adjacent Level Crossing Gate signals to the rear station, as well as information such as IB signal replacement status and Block Proving Track Circuit occupancy. Furthermore, signal control directives originating from the rear station are relayed to the IBH, along with coordinated commands for axle counter reset and inter-station IB reset permissions.
UFSBI is designed to seamlessly interface with various types of interlocking technologies, including Electro-Mechanical, Route Relay Interlocking (RRI), Panel Interlocking (PI), and Electronic Interlocking (EI) systems, offering versatility in field deployment.​​
A typical Dual UFSBI in IBH
A typical Station Installation in IB section
A typical IB Hut Installation
Functional Role in IB Signalling​
In the Intermediate Block signalling architecture, UFSBI serves as the vital communication bridge linking the interlocking system of a station with the signalling setup at the IBH. It is responsible for transmitting and validating essential control commands that drive the IB signal, accompanied by a real-time safety status confirmation. UFSBI provides live status updates regarding signal replacement conditions and the state of the block section track circuits. It also communicates route locking feedback and track clearance indications, all within a fail-safe communication envelope. Through this mechanism, both the rear and forward stations maintain real-time situational awareness and control over train movements within the IB section, thereby upholding operational safety and continuity, which is of paramount importance when carrying human life and materials.
A working Intermediate Block Signalling Scheme using UFSBI & MSDAC on Dual Media on Single Line Section
Functional Role in IB Signalling
In the Intermediate Block signalling architecture, UFSBI serves as the vital communication bridge linking the interlocking system of a station with the signalling setup at the IBH. It is responsible for transmitting and validating essential control commands that drive the IB signal, accompanied by a real-time safety status confirmation. UFSBI provides live status updates regarding signal replacement conditions and the state of the block section track circuits. It also communicates route locking feedback and track clearance indications, all within a fail-safe communication envelope. Through this mechanism, both the rear and forward stations maintain real-time situational awareness and control over train movements within the IB section, thereby upholding operational safety and continuity, which is of paramount importance when carrying human life and materials.​​
Track Detection Using MSDAC
​Overview of MSDAC
The Multi-Section Digital Axle Counter (MSDAC) is a sophisticated track vacancy detection system extensively adopted across Indian Railways. It operates by identifying the presence or absence of rolling stock on a section of track through the detection of train axles. MSDAC offers superior accuracy and is well-suited for deployment over multiple track sections, hence its “multi-section” designation.​
Functional Capabilities of MSDAC
MSDAC relies on wheel detection sensors strategically installed at the entry and exit points of each track section under surveillance. It is engineered for multi-section applications, enabling it to simultaneously monitor numerous contiguous track segments. The system operates in a fail-safe mode, whereby any fault condition defaults to an ‘occupied’ status to preclude unsafe signalling actions. MSDAC communicates seamlessly with the station interlocking system as well as with UFSBI, thereby integrating fully with the signalling logic and contributing to overall system safety.
​
Role in IB Signalling
Within an IB signalling framework, MSDAC plays an indispensable role by accurately detecting the presence and clearance of trains between the stations and the IB hut. It ascertains when a train has fully traversed and vacated the Intermediate Block section, and /or Block section thereby confirming track availability. The axle count data is used to generate a Track Clear (TC) condition, which is then relayed to the interlocking system and to UFSBI for further action. By ensuring that only when all axles have been counted in and out is a track declared vacant, MSDAC enhances the reliability of the block system and ensures that following train movements are authorised only under safe conditions.
​​
Integrated Working of IB, UFSBI, and MSDAC
The coordinated operation of IB signalling, UFSBI, and MSDAC forms the core of a modern fail-safe train control system. When a train requires clearance to proceed, the sending station initiates a line clear request to the receiving station via UFSBI. Simultaneously, MSDAC verifies that the Intermediate Block section is unoccupied and transmits track status information to the interlocking system. This status is then shared over UFSBI.
Upon validation of safe conditions, the IB signal is cleared through the interlocking logic. UFSBI ensures that command and status exchanges between the station and the IBH are reliably completed, with all vital signals appropriately confirmed. As the train proceeds, MSDAC monitors its movement through the section and updates the occupancy status upon the train’s exit. This update is communicated to both the interlocking and UFSBI systems. In the event of a communication failure or detection anomaly, the system defaults to the safest possible state by reverting signals to danger, thus safeguarding against any potential mishap.
A MSDAC Installation in IB Hut
MSDAC Detection Points
Advantages of Using UFSBI and MSDAC in IB Signalling
The deployment of UFSBI and MSDAC within the Intermediate Block signalling environment significantly augments the capacity and safety of railway operations. By allowing multiple trains to occupy what was formerly a single block, the system enhances throughput and efficiency. Both UFSBI and MSDAC operate within stringent fail-safe parameters, ensuring operational integrity even under fault conditions. Their compatibility with diverse interlocking systems and their high reliability metrics, including superior Mean Time Between Failures (MTBF), further support sustainable and resilient railway operations.
​​
Equipment Layout and Media Connectivity plan in IB Signalling using UFSBI & MSDAC on Dual Media
Conclusion
The integration of UFSBI and MSDAC within the Intermediate Block Signalling (IBS) framework marks a significant stride in enhancing the operational capability and safety of Indian Railways. MSDAC ensures accurate and fail-safe track detection, while UFSBI—particularly in its implementation using M/s Deltron products—provides a robust, secure, and cost-effective digital platform for the exchange of vital signalling information. Together, they form a dependable and harmonised system that enables safe and efficient train operations. Despite the emergence of competing technologies over the past 15 years, UFSBI remains the most preferred and trusted solution for vital I/O exchange, with its proven performance across more than 1,000 IB sections standing as a testament to its engineering integrity and enduring relevance across India’s most demanding rail corridors
Zonal Railway Approved Schematics & Guidelines for Auto Signalling using UFSBI & Non-Vital Mux with MSDAC, Part 1
Zonal Railway Approved Schematics & Guidelines for Auto Signalling using UFSBI & Non-Vital Mux with MSDAC, Part 2
Zonal Railway Approved Schematics & Guidelines for Auto Signalling using UFSBI & Non-Vital Mux with MSDAC, Part 3