Control of structural risks arising from track replacement.

Metronet's extensive programme of ballasted track replacements on London Underground lines causes a large number of high-consequence, low-probability structural risks. (The name 'Metronet' has recently ceased being used.) The acceptable level of risk is discussed. Many of the risks involve small temporary adverse changes, and it is shown that these are not acceptable simply because they are small. Economical mitigations of risk, involving reasoning or modest modification of the conditions during the track replacement, are described for situations where calculation alone cannot demonstrate an acceptable level of risk. [ABSTRACT FROM AUTHOR]