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Zipline·Software Engineer·Onsite - System Design / Architecture·Senior

Senior
Jun 2026

Summary

System design round at Zipline for a software engineering role. The prompt was a railroad crossing control system, which sounds niche but actually covers a ton of ground: sensors, actuators, redundancy, failure modes. More depth required than I expected going in.

Questions Asked (1)

Q1

Design a safe and redundant control system for an at-grade railroad crossing, covering train detection, road user warnings, gate control, failure handling, and maintenance support.

System DesignTechnical Trade-offsAdaptability & Ambiguity
Author's notes

This one is deceptively wide.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints (e.g., safety integrity level, traffic volume, environmental conditions) to frame the design. Then propose a layered architecture with redundant components and fail-safe mechanisms, covering detection, warning, and gate control. Finally, discuss failure modes, maintenance, and trade-offs, emphasizing safety and reliability.

Pro tip: Explicitly reference industry standards like IEC 61508 or CENELEC SIL ratings to demonstrate domain awareness, and highlight how redundancy and fail-safe design reduce common-cause failures.

1. Clarify Requirements and Constraints

Ask about safety integrity level (SIL), traffic volume, train speed, environmental factors, and regulatory requirements to scope the design appropriately.

2. Design Train Detection Subsystem

Propose redundant detection methods (e.g., track circuits, axle counters, radar) with voting logic to ensure reliable train presence detection.

3. Design Warning and Gate Control Subsystem

Specify fail-safe warnings (lights, bells) and gate mechanisms with redundant actuators and position sensors, ensuring gates default to safe state on failure.

4. Implement Failure Handling and Redundancy

Use 2-out-of-3 voting, watchdog timers, and fail-safe design so any single failure degrades safely; include diagnostics and fallback modes.

5. Address Maintenance and Testing

Incorporate self-testing, remote monitoring, and scheduled maintenance with built-in test equipment to ensure ongoing reliability.

Key Points to Mention

  • Redundancy techniques: dual-channel architecture, 2-out-of-3 voting, diverse redundancy to avoid common-cause failures.
  • Fail-safe principles: de-energize to safe state, fail-safe gate positions, and warning activation on fault.
  • Train detection technologies: track circuits, axle counters, and their trade-offs (cost, reliability, maintenance).
  • Safety standards: IEC 61508, CENELEC EN 50126/50128/50129, and SIL ratings for railway applications.
  • Failure modes and effects analysis (FMEA) and fault tree analysis to identify and mitigate risks.
  • Maintenance support: built-in test equipment (BITE), remote diagnostics, and preventive maintenance scheduling.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.