For this service 139, after Toa Payoh interchange, it travels to
Jalan Toa Payoh Bus stop 05 which is opposite to Trellis Towers.
After this bus stop, it moves to Jalan Datoh to go to Balestier
Road.
The reason why transitlink bus guide does not mention of this road
because there are no bus stops on Jalan Datoh. Another example will
be service 315. After 315 is moving towards Yio Chu Kang Road from
Serangoon Garden Way, it uses Yio Chu Kang Road to go to Serangoon
North Avenue 1. However, the transitlink bus guide also does mot
mention that service 315 uses Yio Chu Kang Road to go to Serangoon
North Avenue 1.
Next, I think the reason why service 139 uses Jalan Datoh instead
of going to Thomson Road and then to Balestier Road because that is
a shorter route than the latter.