I remember when I visited Kuala Lumpur railway station in 2018, it was extremely deserted.
Even the commuter 'KTM Komuter' trains were something like 1tph off-peak and 2tph peak, and the station was significantly empty most of the time. I don't remember seeing freight.
KL Sentral station was no fun to visit, it's newer and all underground, like a smaller version of B'ham New Street.
You may want to check out wiki for KTM Kargo:
en.wikipedia.org
It says "there are 45 cargo train services daily with 23 routes daily", so maybe that's why I couldn't see any when I was there. The link above has some routes as well, although I'm not familiar with them. You may have to use Google maps to figure out the intermediate points.
For something interesting, you can go to Muzium Negara (National Museum). At the entrance (without entering the museum), there is an English Electric KTM Class 22 loco:
en.wikipedia.org