If you want to travel on the TBS to Penang train, you have a couple of options available.
1) Take a direct ETS train from Bandar Tasik Selatan Railway Station (adjoining the TBS Bus Terminal).
2) Take a KTM Komuter or KLIA Transit to KL Sentral, then take one of the more frequent trains from there.
There are currently only two ETS services a day that stop at Bandar Tasik Selatan travelling north.
One travels to Butterworth in Penang (located 15 minutes away by ferry from Georgetown), the other to Padang Besar.
Train EG 9322 departs BTS at 17:14 - Arriving in Butterworth at 22:02
Tickets prices (harga tiket) for the direct train from TBS to Butterworth cost:
Adult fare : RM 60
Child fare : RM 34
If you arrive at TBS bus station in the early morning, you could take the ETS train from TBS to Parit Buntar (for commuter connections to Butterworth).
Train EG 9420 departs BTS at 09:44 - Arriving in Parit Buntar at 14:09.
You than connect to the KTM Komuter Utara train service (northern commuter train) to Bukit Mertajam (3 stops - 20 minutes trip duration), then change trains there for Butterworth (the train departs 5 minutes after arriving at BM, then it's a 2 stop - 10 minutes trip duration to Butterworth).
Another option is to take a cheap, regular KTM Komuter train to KL Sentral Station (18 minutes trip duration) or one of the slightly more expensive KLIA Transit trains (10 minutes trip duration).
From KL Sentral to Butterworth Penang there are more frequent services (don't take the late trains as the ferry stops running just after midnight).
Before contemplating taking this route, make sure you can get a seat on the ETS train from KL first.
You can book your ETS tickets departing KL Sentral from the KTM ticket counter at Bandar Tasik Selatan Station.
If all these trains are full, or do not depart at the required time, you could always take the option of taking one of the Padang Besar bound trains and getting off at Parit Buntar, then continuing the journey by KTM Komuter train as described above.