I did the overnight train about 5 years ago both ways.

It was a good way of saving on accommodation. Train was comfortable enough, sort of like Thai trains (non-compartment). It was all good but for the very early morning (6am) arrival into Singapore and fact you had to get out and all your stuff for immigration and then back on the train. On the return, you complete Malaysian immigration at the station before departure in Singapore (apparently technically, the rail line is Malaysian territory)

I remember if you booked your train on the ktmb site, you paid in RM for your journey which at the time was cheaper than paying in SGD