SINGAPORE - Rail operator SMRT warned that there may be delays on the Circle Line (CCL) on Thursday evening (Nov 3) because of an unresolved signal interference.

It tweeted at 6.07pm, saying: "Due to intermittent signal interference, CCL trains may experience delays.

"For safety, CCL trains are manned this evening. We are sorry."

The unknown interference resurfaced on Wednesday morning (Nov 2), after a two-month reprieve. It first appeared in late-August, and crippled Circle Line service for about a week. It disappeared as suddenly and as mysteriously as it appeared, and engineers were not able to trace its source.

SMRT suspects it may have to do with telecommunications cables in the tunnels, but despite having roped in the Infocomm Media Development Authority and mobile network operators to help, it could not resolve the issue.

The stray signal interferes with the wireless signal communication between the trains and the tracks, causing the trains to apply emergency braking.