Sorry to burst your bubbles guys and gals who are hoping for
DD's in Yishun esp 855/857 (though I would like to see them have
DD's).
There are a few very low lying branches along Yishun Central 1
which have not been trimmed, so until that is done, the signs are
that 855/857 are not slated to have DD's (along with 853 which is
obvious). If there are any DD services in Yishun Interchange from
SMRT side then it will be 854/856.
Looking at SMRT's history of having 851 as its most
favouritely-neglected high load trunk service, it doesnt look like
851 will get DD's any time soon, probably swapping the 6 bendies
for maybe 3 DD's if any. Or maybe removing all bendies and have
full SD fleet. Its pretty disgusting how SMRT had been running
851.
If the branches do get trimmed one day then perhaps 855/857 are
going to get DD's imminently.
Looking toward Central 2 - see the wad of branches almost
touching car level reaching out nearly to the centre divider:

Perspective view with a bus for height comparison

Location of this shot is just outside Yishun Temp
Polyclinic.