Commuters along the North-South Line were told to add 20 minutes of travel time between Marina South Pier and Jurong East on the North-South Line during yesterday evening's peak hour.

Photos sent to The Straits Times showed long queues at Raffles Place and Newton stations. On social media, others wrote of delays at stations including Jurong East, Ang Mo Kio, Yio Chu Kang and Bukit Batok.

SMRT said in a tweet at 6.47pm: "Due to new signalling system checks, pls add 20mins travel time between #MarinaSouthPier & #JurongEast both bounds. We are sorry." It later said free regular bus services were available at certain stretches on the train line.

At 7.17pm, SMRT said trains were progressively returning to normal speed and asked commuters to expect 15 minutes' extra travel time between Marina South Pier and Jurong East. At 7.41pm, it said train services were returning to normal.

IT executive Kelvin Ong said he was on a train from Kallang at 6.10pm when he saw a crowd at City Hall. He took another line at Raffles Place but "it was even worse".

Mr Ong, who usually takes about 40 to 45 minutes to reach his Woodlands home, was at Bukit Batok station when he spoke to ST at 7.20pm. He said the situation was "chaotic", adding: "I'm not sure what they are testing. At this hour, it's not really convenient."

Student Keith Chong, 23, told ST he took a bus after seeing a large crowd at Bishan at 6.50pm. He said there were announcementsmade and "a lot of staff guiding people".

Another commuter, Mr Eugene Mok, 32, said his trip from Yew Tee to Bukit Batok took 48 minutes, when it typically takes eight minutes.

Testing on the upgraded signalling system began in late March. The Land Transport Authority and SMRT previously said the new system will allow trains to run at shorter intervals of up to 100 seconds, instead of 120 seconds during peak hours. It was also unveiled as one of the key projects to renew the 30-year-old North-South Line.

Christopher Tan and Lydia Lam