Track circuits and welded rails: How is continuity of electric current maintained across welded rail joints?

Introduction / Context:
Track circuits use the running rails as conductors for train detection and signaling. Reliable electrical continuity across joints is essential, especially with welded joints or insulated joints placed for block boundaries.

Given Data / Assumptions:

• Continuous welded rail track with occasional insulated joints as required by signaling design.
• Need to maintain low-resistance return path for current.

Concept / Approach:
Bond wires (rail bonds) are employed by welding or brazing their ends to the web/foot of adjacent rails, providing a robust, low-resistance parallel path that survives vibration and weathering. This ensures circuit reliability even if the physical joint area has higher resistance.

Step-by-Step Solution:
Identify the need: continuity across joint areas for signaling current.Provide rail bonds: weld a flexible copper bond wire to both rails across the joint.Verify continuity and resistance meet signaling specifications.

Verification / Alternative check:
Routine testing with ohmmeters confirms low resistance across bonded joints; visual inspection ensures weld integrity and strain relief is adequate.

Why Other Options Are Wrong:

• Insulated plate underneath rails: provides no electrical bonding; it insulates from sleepers or baseplates, not rail-to-rail.
• Insulation in expansion gaps: increases resistance rather than maintaining continuity; used only where isolation is desired.
• None of these: incorrect because bonding wires are standard practice.

Common Pitfalls:
Using mechanical clamps instead of welded bonds in high-vibration areas; neglecting redundancy bonds on vital circuits.

Final Answer:
welding ends of a wire to each rail