Transport Minister Suriya Jungrungreangkit blames Pink Line's conductor rail collapse on Sunday morning on soil dropped from a lifted sheet pile at the system's construction site.
Inspecting the scene on Sunday, Mr Suriya said officials believed that soil dropped from a lifted sheet pile might have fallen onto the tracks along the Pink Line. A patrol train that normally runs from 4am every day then is thought to have run over the soil, causing the train to shake and pull the conductor rail from its clamps.
The minister said that a crane was used to lift sheet piles from the construction site of the Pink Line system to reopen traffic lanes earlier closed for the monorail construction.
The rail – about 10 centimetres wide and 5cm thick – fell along a distance of about four kilometres on Tiwanon Road from Khae Rai intersection to Chon Prathan market in Nonthaburi province.
The Department of Rail Transport would check the safety of the system in seven days and officials concerned would then consider reopening the Pink Line, Mr Suriya said.
"If such an incident recurs, there will be punishment as required by the contract," the minister said.
Pakapong Sirikantaramas, governor of the Mass Rapid Transit Authority (MRTA) which supervises the Pink Line, said the patrol train weighed about 100 tonnes and when it shook because of the dropped soil, it pulled on the conductor rail very hard.
It would take about seven days to pick up the fallen conductor rail and reinstall it. The task would be done at night to enable traffic on Tiwanon Road to run during the day, he said.
Pending the reinstallation, the Pink Line could resume service at 23 stations between Chaeng Wattana and Min Buri stations on Monday except for the affected section between Nonthaburi Civic Center and Pak Kret Bypass stations where there were seven stations altogether, Mr Pakapong said.
The MRTA governor promised to compensate all those affected by the collapse. It was reported that four vehicles were damaged and a tall power pole was left standing at an angle leaned because a section of the conductor rail fell on its power cables.
The MRTA started a free trial run of the Pink Line on Nov 21 and its passenger traffic peaked at about 100,000 passengers a day on Saturday. The state enterprise planned to start to collect fares on the system on Jan 3.