Reader Gabriel Tham wrote in to ask: "Why do MRT trains have to move slower when it rains? Road users drive slower for safety reasons, but why trains?"

Senior transport correspondent Christopher Tan answers.

MRT trains go slower when it rains for safety reasons too.

However, the weather has an impact only on trains running on surface lines. This is because, when it rains, the tracks get wet. And trains require a longer braking distance when the tracks are wet - just like land vehicles on wet roads. To compensate for the longer stopping distance, trains go slower.

The rain does not affect underground lines. But operators might at their discretion adjust scheduling of such lines, to cater for the slower arrivals of surface trains at interchange stations on wet days.

The train network is like a giant living organism - what happens to one part will have an effect, no matter how small, on other parts. The commuter does not always feel this ripple effect unless it is major.