Hi Debbie P
We live in Paremata near Porirua harbour and I often take the train to work - less problems than the Johnsonville line but you get the occassional delay as with any transport system. Plimmerton isn't quite on the Kapiti Coast - the Kapiti starts further up but the train line (Paraparaumu line) is the same for both.
Train to/from Plimmerton-Welly is about 30 mins (20ish mins from Paremata). Driving is a bit worse whilst they're messing around with the roundabout road works at Paremata so it might be worth training it from Plimmerton (although when they've finished the roadworks I don't reckon its going to be any better). Because we live in Paremata we can take a back road route and bypass the roundabout - something that put us off living over the bridge in Mana/Camborne/Plimmerton or Kapiti coast - we spent 3 weeks in Mana in a motel when we moved here - because that was where I wanted to settle - after a week (during school holidays) we realised what a nightmare the traffic was around the Paremata roundabout and decided to live on the other side of the bridge! Driving takes about half an hour from Paremata - I'd add 10-20 mins for Plimmerton depending on traffic flow.
My advice is try it out before you commit yourself to definitely renting/buying there.
That said Plimmerton is lovely - we go to the beach there most weekends for walks - house prices are more expensive there but the sea views are lovely.
Good luck!