The KTM Komuter system is an inter-city rail line that mostly deserve Kuala Lumpur and the surounding Klang Valley. It is reputed as being quite slow, with trains at roughly 15-minute intervals during peak and 30 minutes off-peak. It also is notorious for cancellations, delays, breakdowns and overcrowded cars during peak hours. On the plus side, the trains are air-conditionned and the fairs are very reasonnable.
In order to avoid the queues, you can get a Touch'n Go card, which you can load with any amount of value and then use on the KTM Komuter, RapidKL LRT, and KL Monorail systems as well as motorway tolls and in many car parks.