A big problem you have is that the state has lost a lot of the wherewithal to build public infrastructure, having privatised everything during the neoliberal revolution of the 80s. You can see this here in Ireland. We're pretty good at building roads; there are always roads being built. But we haven't built big infrastructure projects like hospitals or railways, and so they often end up being expensive disasters with a great degree of rent seeking going on. Interestingly, Madrid built a metro system very successfully by effectively designing the construction around the election cycle so politicians could 'cash in' on various tranches of the metro being completed (say a section built every 5 years, lined up with election cycles). A pragmatic solution!
I'd argue the tide has shifted, at least in Britain (and many other European countries), toward the state having a more significant role in the ownership of public goods. The slow beginnings of this can be seen in the UK with renationalisation (the clunkily named Great British Railways) along with the constant backlash against private water, utility companies etc.
Britain suffers the pioneers curse. First to do it, stuck with ancient infrastructure (Same as ourselves!) and thus restricted by it. It comes with advantages of course, they have an enormous expanse of underground tunnels since the 1860s, whereas some 50 years after proposal, all we have to show for our metro is part of the airport designed to accommodate and 500 million euro spent on nothing.
Onto the Enterprise. The Belfast main line is one of, if not the busiest line in the country, with its southern terminus being the most busy station in the country. With major improvement works comes major delays, so on and so forth. Ultimately, I think they may have to just bite the bullet and go either quad track or with a separate HSR running in parallel. Both have advantages and disadvantages, but it certainly cannot continue as is. Widening the viaduct up towards Clontarf and onward would be a 19th century slightly suspect railway developers job!
The US is a different kettle of fish of course.