What Happened: A Break in the Current
On August 19, 2025, amid heavy rains and suspended local train services, Mumbai’s monorail bore an unexpected load surge. Two monorail rakes stalled—one between Mysore Colony and Bhakti Park, and another further down the line—stranding nearly 800 passengers.
The cause? Excessive overcrowding. The trains were designed for a maximum load of ~90–100 metric tonnes (including passengers, body weight, and equipment). But with panicked commuters crowding aboard, the weight exceeded 109 tonnes.
This overload broke electrical contact between the third rail (power rail) and the train’s collector shoe—the system that transfers high-voltage DC electricity to run the motors. Without this connection, the train’s propulsion and onboard systems shut down.
The Science Behind the Glitch
- Monorail Power Supply:
Mumbai’s monorail uses a 750 V DC third-rail system. A conductor rail runs along the track, while the train’s shoegear (a spring-loaded device) maintains pressure against the rail to draw current. - Why Overcrowding Caused Failure:
- Weight Shift: The extra weight increased downward force on the bogie (wheel assembly), changing suspension geometry and causing the collector shoe to lose contact with the rail.
- Vibration Amplification: Overloading intensified vibrations and oscillations, making the current collector bounce—leading to intermittent arcing, heat, and eventual disconnection.
- Thermal Stress: Power draw increased sharply due to higher load, causing overheating of conductors, which tripped protective relays.
- Ventilation Problem: With the power off, HVAC (heating, ventilation, and air-conditioning) failed. The monorail cabins are relatively airtight for noise and safety reasons, so oxygen levels dropped quickly. Passengers experienced hypoxia-like symptoms (shortness of breath, dizziness, fainting).
Why Towing Wasn’t Possible
Unlike metro systems, the Mumbai monorail has narrow guideways and complex turning radii. With the rake overloaded, towing risked structural stress on joints and bogies. Engineers opted for emergency evacuation using cranes and sky ladders—a slower but safer method.
A Look at Mumbai’s Monorail Engineering
- Track System: Straddle-beam monorail, where trains "sit on top" of a single concrete beam, stabilized by rubber-tired wheels gripping the sides.
- Speed & Capacity: Designed for 31 km/h average speed, carrying about 20,000 passengers/hour at peak.
- Weak Point: The system was never meant for surge ridership like the city’s heavy-use suburban trains, which can carry 60,000 passengers/hour on a single corridor.
Lessons from the Breakdown
- Load Limits Must Be Enforced: Exceeding design capacity doesn’t just slow operations—it creates mechanical and electrical hazards.
- Need for Better Power Systems: Upgrading from a third-rail to overhead catenary or redundant collector shoes could reduce contact loss.
- Emergency Ventilation: Automatic battery-powered ventilation fans could prevent suffocation in future incidents.
- Integration with Metro: Direct linkage with Mumbai’s upcoming Metro Line 3 may distribute commuter loads better, reducing pressure on the monorail.
The Road Ahead: Science Meets Policy
The Mumbai Metropolitan Region Development Authority (MMRDA) has announced:
- Doubling the fleet from 8 to 18 rakes by August 2025.
- New rakes equipped with CBTC (Communication-Based Train Control) for automated operation.
- Improved fire safety, larger capacity (10% more per coach), and energy-efficient systems.
- Integration with Metro and upgraded depots to streamline maintenance science.
Conclusion
Mumbai’s monorail was built as a modern, eco-friendly alternative to congested road and rail systems. But as this breakdown showed, engineering designs have limits—and ignoring them risks lives. With scientific upgrades and smarter planning, however, the monorail can still evolve into a safe feeder network for India’s financial capital.