If you can get the express train (which usually only has 2-3 stops between Butterworth and KL) and it has a business cabin, I'd go for that. Do check the George Town - Butterworth ferry schedule. IIRC, it leaves every 30 minutes (on the hour and half past) outside of workday peak hours. Takes about 10-15 minutes to cross the channel and another 5-10 mintues walk from ferry terminal to Butterworth train station. If you are staying in George Town and don't have a lot of bags, I'd recommend taking the ferry across to Butterworth instead of Grab (uber) as traffic on the bridge can be unpredictable and it's a huge detour.

All platinum trains have a business cabin whilst all gold trains do not. Gold trains have more stops including more local stations. The business cabin tend to sell out first before the regular cabin and I'd advise you to book 2 weeks in advance if you are traveling on a weekend; earlier if it's school holidays or long weekend.

If you have time, maybe hop off at Ipoh for a night or two (about half way between Butterworth and KL). It's a nice small city with karst hills and good food (you can also take a day trip to Cameron Highlands if you want).

Do buy your tickets or get off at "KL Sentral" instead of "Kuala Lumpur". "Kuala Lumpur" is the old train station, which is about 1 km closer to the city but not the closest to a public transit.

There are some nice scenery on the train but it's not to die for. Mostly jungles, suburban homes, and oil palm plantations. The ride between Taiping and Ipoh is pretty tho where you'd pass through the main mountain range (and just a bit north of Taiping, you'd cross a huge lake and the train passes through the middle of the lake. Bukit Merah on the map).

If you are flying, maybe fly into KL-Subang (SZB) airport instead of KL International (KUL). Subang is much closer to the city but if you arrive during office rush hours, traffic can be really bad.

Hope this helps.