Having trains running in circles are not practical.
If you have a line in continuous loop, there is no termination
station. Terminals are important. Trains stopping at terminal
stations stop for a while, as you probably know.
Whenever there is a delay somewhere on the line, it will take
forever to remedy and recover as there is no waiting at terminals.
Trains pile up behind, leaving a black hole ahead of the delay, and
there is no way to recover time, as there is no time left to
recover.
A solution is for trains to run in circles, but, stop somewhere
along the line, and then reverse direction.
Paya Lebar is the best choice, terminate at middle platform for
trains going clockwise, trains going anti-clockwise terminates at
Promenade Platform B. While Dhoby Ghaut bound trains terminate at
Paya Lebar Platform B and Dhoby Ghaut. Platform A in Paya Lebar
unused, maybe for peak hours where trains go round the circle and
terminate there instead of Promenade.
[EDIT]
Here's a diagram to help you understand better
Green CCL Service will only use Platform B.
Also, I made a mistake at Paya Lebar, green CCL continues from
Blue CCL, Blue CCL terminates at Platform A, then train goes to the
middle platform as DNB, then to the Platform B, before continuing
it's journey.
