Complex sentence: Since then, the C151A has not been on the North South Line, except that it only comes back to Bishan depot if the train need major matainence overhaul, that Changi and Ulu Pandan depots cannot provide.
Split into simpler sentences: Since then, the C151A has been temporarily withdrawn on the North South Line. The train only comes back to Bishan depot if the train need major matainence overhaul, that Changi and Ulu Pandan depots cannot provide.