Originally Posted by Colm Moore
In that case, I think a lot of it is down to speed - service speeds in the north are much slower.
General track condition in NI is at least as good as Irish Rail, apart from the Larne line east of Whitehead and the Derry/Londonderry line, west of Coleraine. On the whole both under and over bridges tend to have been more recently renewed, often for a higher axle-loading than on Irish Rail.
As for the riding of the Enterprise coaches, crossing the border is immediately obvious, especially in the southbound direction when most trains are travelling at 90mph. I think the issue is not so much the condition of the the track as the profile of the rail head which in the Republic follows a German standard while NIR follow a British standard.