Train types - what is MK1, 2, 3, 4?

[Mr T.]

I might be asking a question that's been explained in other threads/posts, but can someone explain what the train types are that people are mentioning in their posts, specifically MK3, MK4, etc.

I've always described them under the following categories:
Harry Potter - the ones that have steam coming out of them and are occasionally used in movies depicting the 1920s.
Intercity - the 'original' intercity trains with the automatic doors
Green seats - the newer than harry potter but older than intercity ones with the green seats and brown leather headrests.
Refurbished - kind of like the intercity but with slam doors and windows that open in the carriages.
Commuter - similar to intercity but have windows that open in the carriages
Arrow version 1- aka barrow: original arrow trains that have no head rests and hurt your arse when you're sitting on them for too long.
Arrow version 2 - the arrows with headrests, yellow bars and loads of standing room.
Cu Na mara - any train with tinted windows, big seats and air conditioning that works. Usually goes to Galway.
Dem fancy trains or 'Bullets' - the new Cork train with the bullet nose. (Bullet does not attempt to make any reference to speed.)

Thanks.