Detailed overnight inspections turned up more clues on Sunday of
what might have gone wrong on the MRT system's critical 'third
rail', which supplies electrical power to the trains.
Metal contact devices - called 'collector shoes' and attached to
the trains - glide below the third rail, picking up power to keep
the trains running.
Initial investigations after last Thursday's major breakdown
indicated something had gone wrong with a section of the third
rail, and the collector shoes on four trains were found
wrecked.
After the MRT system crashed again on Saturday, an extensive
inspection was done on Sunday along 93km of track on the
North-South and East-West lines.
A total of 122 Land Transport Authority (LTA) and SMRT employees
combed the tracks, scrutinising the third rail. Another 40
inspected the collector shoes on all 123 SMRT trains.
On Sunday, LTA officials highlighted a new item, the metal grips -
or 'claws' - that help to keep the third rail stable.
These claws appear at regular intervals along the third rail.
Sunday's inspections found that several had fallen off.
There is a possibility that once they were dislodged, the
unsupported portion of the third rail may have become misaligned or
sagged over time. That, in turn, could have damaged the collector
shoes of passing trains.
But what caused the claws to fall off in the first place?
LTA officials on Sunday floated a preliminary theory that the claws
might have been affected by vibrations from some sections of the
tunnel tracks. These sections of the track sit on floating slabs
below which are bearings that act like shock absorbers.
The 'floating slab tracks' are installed in heavily built-up areas,
and help prevent the vibrations from passing trains from affecting
buildings in the area.
In an unprecedented move, the LTA delayed MRT services on the
North-South and East-West lines by several hours on Sunday for the
track inspection.
LTA and SMRT staff also inspected the underframes of MRT trains,
and checked their alignment with the third rail.
LTA group director for rail (Thomson and existing lines) Chua Chong
Kheng told reporters in the afternoon that all the dislodged claws
had been put back in place and secured firmly to the third
rail.
Eighteen of the 21 dislodged claws were found along the southbound
track of the North-South Line. Two were found in the northbound
direction, and one on the East-West Line.
The inspection also found 34 damaged third-rail covers - 28 on the
southbound tracks and six on the northbound ones.
The third-rail claws might have been dislodged by vibrations from
portions of tunnel tracks with floating slab tracks.
Mr Chua said most of the missing claws were found at sections of
tunnels in the city area where there are floating slabs.
But he stressed that nothing was conclusive yet.
'We have to further review and evaluate the matter, but this gives
us some leads into solving the problem,' he said.
He added that the
shock-absorbing bearings below the floating slabs are designed to last 50 years. The
SMRT inspected a sample
10 years ago and found the bearings in good
condition.
For now, the LTA has applied a temporary speed limit of 40kmh in
sections of the track with floating slabs.
Transport Minister Lui Tuck Yew, who chaired the media briefing on
Sunday, said: 'There is now some indication that we ought to focus
more of our attention and analysis on the floating slab track.'
The LTA intends measuring vibration levels at those sections, and
also plans to retrieve and check samples of the bearings below the
floating slab tracks to see if they are still in good
condition.
Noting that this would not be a straightforward process, Mr Lui
said the operation would likely require more than the four hours or
so between revenue service each day, meaning another longer closure
is likely.
He said train speeds in the affected areas would be progressively
increased when 'it is appropriate to do so'.
