one more thing, if you take the train option, be prepared to spend 2 days. And its not some nice luxury train unless you do this
Eastern and Oriental Express - Luxury Train travel through South East Asia, Thailand and Singapore. and pay a bomb

you could consider flying airasia from HKT to KUL and down to SIN. I'm not sure about the loads of HKT-KUL on MH but I can ask around if you want.

And for KUL-SIN leg, if you want to save a few pennys, you could do KUL-JHB and bus over to SIN or the other way round. As usual, KUL-JHB is a "corporate route" people fly in the morning for meetings and out in the evening. So try only noon flights. I'm not sure if you can save on ZED for KUL-JHB but ID for me is cheaper than my bus fare home from the airport. The airasia does a reasonable rate for this route too