Ballast maintenance practice Where is ballast packing primarily carried out to provide support and maintain track geometry?

Introduction / Context:
Ballast supports sleepers and enables drainage while allowing geometric adjustments through packing and tamping. Understanding where packing is focused helps in appreciating maintenance operations and the mechanics of track support.

Given Data / Assumptions:

• The options distinguish different locations around sleepers and rails.
• We assume conventional ballasted track and standard tamping procedures.

Concept / Approach:
Ballast packing is primarily performed beneath sleepers to achieve proper bearing and correct levels, cross-levels, and alignment. Although shoulders and sleeper ends are also important for lateral resistance and stability, the essential act of 'packing' to correct levels happens under the sleeper.

Step-by-Step Solution:
Recognize the purpose of packing: restore and maintain geometry by supporting sleepers evenly.Identify the critical contact zone: the sleeper–ballast interface below the sleeper.Select 'Under sleepers' as the primary locus for packing.

Verification / Alternative check:
Track maintenance manuals and tamping machine operations describe insertion of tines and compaction under sleepers as the core process of packing.

Why Other Options Are Wrong:

• Near ends/shoulders: important for stability but not the principal definition of packing.
• Between rails only: packing is not limited to this region; it must support each sleeper fully.

Common Pitfalls:
Equating shoulder maintenance with packing; shoulders resist lateral movement, while packing under sleepers primarily corrects geometry.

Final Answer:
Under sleepers