Can Anyone Tell Me Why a Heavy Rail Link to Dublin Airport Can Not be Built in the Short Term?

If / when the real impact of the price of aviation fuel and travel carbon taxes are felt and only a matter of time, then demand will drop to such a level that the existing links will be excessive.

Very unlikely. This is one of the stupid ones that climate change activists focus on. There is no realistic substitute.

Very simply put, because the government doesn't want to.

A more important reason is WHY don't they want to. And here you have lots of stuff, I'll put a few thoughts below.

1. The Irish public service in general doesn't like providing services, every new service has to be fought tooth and nail for.
2. Some vested interest doesn't want it as it might decrease their profits.
3. In the current government, the Greens are concerned that a blade of grass somewhere might be disturbed by the project.
4. Someone with a connection to somebody has property in the way and doesn't want it to be affected.
5. Somebody with a connection to someone doesn't own property on the route, and so they won't make a killing as their property won't compulsorily purchased, but if a substantial deviation to the route was included they would.

No evidence for that.

This is one of the last things that governments will properly disrupt because of the lack of alternative options.

I wouldn't be so sure.

Back in the day, I had myself convinced that electric cars could never practically replace internal combustion vehicles (power density, weight and all that) nor that batteries could have any role in utility scale storage (way too expensive).

Both these predictions turned out to be spectacularly wrong. I had underestimated the unstoppable power of mass production - in this case that of batteries - leading to a 97% price decline in 30 years (and seemingly continuing - CATL or BYD recently announced a 40% price cut for some of their battery lines later this year) and constantly improving chemistries. EV range is doubling every 7 years which means they are constantly eating into applications currently only possible with ICE.

One trump card for electric is that the cost of "fuelling" an EV is a fraction of that of buying fuel for ICE vehicle. And the aviation industry is fixated with fuel costs - so there is absolutely no way I would bet against electric aviation being a thing within 10 or 15 years particularly for shorter flights - say less than 90 minutes.

Like with cars, there will be niche applications where the tech will not initially work (medium and long haul), but it's a mistake to think that just because the technology cannot do 100% of what a previous technology could do, it won't succeed. The fact that horses can do lots of things that cars can do didn't stop Henry Ford.

Perhaps, perhaps not. Fact is that requires a lot of energy to shift metal boxes around in the sky filled with humans going on day trips and weekend breaks and that all eats up resources in one form or another. It's going to need a big jump in ideas and physics to keep this show on the road. I don't think flight is going away but just that it'll become more the preserve of the wealthy & military etc

Small electric aircraft are already available to purchase.

Up to 3 or 4 years ago, the consensus was electric could scale at most to a 20 passenger aircraft with a range of about 200 miles - anything beyond would require hydrogen, biofuels or syngas. Now there are companies working on 90 passenger/500 mile electric aircraft - admittedly at an early stage of development - but it shows their engineers think it's feasible.

And of course, all these companies working on such aircraft realise that weight is their biggest issue. I saw one calculation that with current tech, it would take 35T of batteries to power a fully loaded 737 (about 70T).

Sounds hopeless until you appreciate the power of exponential growth - both in terms of affordability and performance.

Battery performance, along the most important metrics such as power/weight ratio, has improved more slowly but still exponentially - roughly 8% a year over the last 20 years or so. If this trend continues, then the 35T of batteries required to power a 737 will fall to 17.5T in 7 years time, 9T in 14 years time, 4.5T in 21 years, etc. You can see where this is going.

In terms of costs (the bottom line for commercial aviation), battery prices have decreased exponentially over the last 3 decades - now costing 3% of what they did since li-ion started to be mass produced - representing nearly 14% price decrease year-after-year for 30 years. I see no obvious reason why this trend/progress would suddenly stop at this point in history.

You've just described Metrolink, no?

Nope. We have the technology now.

It isn’t happening.