Your fantasy 175 & 138 may only become a reality if SBST and
SMRT merge together....
If SMRT extends 175 to Sengkang, SBST will scream. (And SBST is
unlikely to let SMRT park its buses in Sengkang Int, just like 965
which loops at Sengkang.)
If SBST extends 138, SMRT will scream. (SMRT will probably tell you
to take 171 from Mandai to Bukit Panjang, and take 176 to Bukit
Batok & Jurong East.)
Or you can pray that SBST and SMRT sign agreement and introduce
"joint services", just like Hong Kong's "Tunnel bus services" where
a bus route is jointly run by 2 companies. (The buses of the 2
companies come in an alternate pattern, and the profits generated
from the route are split equally into 2.) But given the history,
the nature, and the politics of Singapore's transportation, such a
"joint service" is unlikely...
