The line via Crumlin is maintained well and is still used for passenger diversions once or twice a year when the line out via Mossley West is closed. Whilst it may be quicker from Belfast, it lost the quicker connections from Newry, Portadown and Lurgan, and direct journeys from Lisburn. The line also passes within spitting distance of Belfast International, which could open up a whole realm of prospects for the line.
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However looking back at WatcherZero's post I believe they could have been inferring more towards the Republic - places like Docklands, M3 Parkway, Nenagh, and the WCR etc have all fallen relatively flat on their faces within the last five years, let alone decade. For the last decade also services have been chopped and changed constantly year after year to save costs and give an optimum servgice on as least trains as possible.
The initial post which cited railways in Ireland was, I believe, Gareth Marston's, and this referred specifically to Northern Ireland railways as an example of a nationalised railway in the UK which has experienced growth.