AFAIK The MKIV carriages are good to 200kph, but the locomotives are capped at 160kph.
I suppose if the speed limit has been raised, it would be easier to say there would be a fast return on investment on new locomotives.
There are still vast sections of track (e.g. Cork-Mallow) where 160kph isn't possible, so I would assume 2 hours is the best case scenario.
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