Rail & Metro: Rolling Stock, Adhesion and Braking
BEML builds metro cars, EMUs and mainline coaches. The rail and metro section tests rolling-stock fundamentals: adhesion-limited tractive effort, braking distance, bogie and suspension arrangement, and signalling interfaces such as CBTC and platform screen doors.
Core rules
- Adhesion limit: Fmax = μ × axle load × number of axles × g. Installed traction power cannot exceed what wheel-rail adhesion permits, particularly in wet or leaf-fall conditions.
- Braking distance = reaction distance (v × t) + braking distance (v² ÷ 2a). Convert km/h to m/s by dividing by 3.6.
- Gauges: Indian broad gauge 1,676 mm; standard gauge 1,435 mm used on most Indian metro systems; metre gauge 1,000 mm.
- Bogies: primary suspension at the axlebox isolates high-frequency track irregularity; secondary air springs control ride and body roll; anti-roll bars limit roll.
- Regenerative braking recovers energy above a threshold speed, blends into friction braking at low speed, and fails safe to friction.
- CBTC gives moving-block headway; platform screen doors interlock with train doors and the signalling system.
Worked example
A 4-axle metro car at 16 t axle load with adhesion μ = 0.15. Maximum tractive effort = 0.15 × 4 × 16,000 × 9.81 ≈ 94 kN. For emergency braking from 80 km/h (80 ÷ 3.6 = 22.2 m/s) at 1.0 m/s²: braking distance = 22.2² ÷ (2 × 1.0) ≈ 247 m; add 2 s reaction at 22.2 m/s ≈ 44 m, giving ≈ 291 m total. Braking is also adhesion-limited, so demanding more than μg deceleration is unachievable.
Traps that cost marks
- Forgetting the km/h to m/s conversion, which inflates or deflates distance by 3.6 or its square.
- Quoting braking distance without the reaction distance.
- Assuming installed traction power equals achievable tractive effort.
- Mixing broad gauge with standard gauge in metro specifications.
- Ignoring the low-speed cut-off of regenerative braking.