Neither the number of major train disruptions, nor the average distance between breakdowns, is a good indicator of rail reliability ("Measuring rail reliability"; Thursday, and "Rise in major breakdowns but MRT gets more reliable"; Tuesday).

The use of only these statistics shows an undue focus on the health of trains over the welfare of commuters. At the end of the day, how does the number of kilometres the train runs before breaking down affect commuters?

Instead, the Land Transport Authority (LTA) should look at the overall cost to the economy of breakdowns.

For example, how many man-hours of delays were incurred? How many people were delayed by more than five minutes? How many people were forced to take alternative modes of transport? How much did they have to pay?

A breakdown late at night has a different impact from a breakdown during morning peak hours.

The former affects fewer people and has a smaller impact.

These metrics also better translate into how people perceive the reliability of the system.

When more MRT lines are built, the extra redundancy in the system also means that there may be alternative routes even in the event of a major disruption. Fewer people will be severely delayed.

Using the new metrics, we can more accurately measure the benefits of extra redundancy in the system.

There will not be an endless and increasingly costly pursuit of near-perfect reliability.

In short, when discussing rail reliability, LTA ought to be sensitive to how rail reliability is perceived and felt on the ground.

This will help operators focus on improving customer experience, rather than technical perfection, which is impossible.

Sum Siew Kee