Hmm... What is your definition of "old" because there are new
locomotives such as the Dalian Class 29 which are also diesel
locomotives. Anyway, all intercity trains must use diesel
locomotives since like what Interception_7 said, not all areas are
electrified. The only electrified areas are Seremban to Ipoh and
Port Klang KTM Komuter station to Sentul.
I mean why are they not using the Blue Tiger the locomotive? If one
day they can electrified the whole line just the Taiwan the Taiwan
高铁, almost all the lines were interconnected then It would offer
another alternative to budget airlines.
At the moment they are still slow and the fare is not cheap. If
from Singapore to Butterworth it would take more than 12 hours!