YES, but the 'western' connection is not often used.
The actual triangle is quite large - a few kms around each side in fact.
According to the station-master a couple of years ago the western
connection is used occasionally to 'put away' a train for crossing if
Tarcoola loop is full, or for ballast trains from the CAR heading out West
on the TAR.
I think Tarcoola station is now unattended (as of May this year). All
signals/track controlled remotely from either Port Augusta or Adelaide (I'm
not sure which).
Kind Regards,
Leo.