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This was always going to be a tough day in the office for the UK Government.

Critics are lining up at Westminster and in south Wales to have their chance to lay into the decision to scrap the electrification of the line west of Cardiff.

I was in the pouring rain exactly five years ago jumping on a train in Swansea with the then Welsh Secretary Cheryl Gillan when the original decision was announced.

Since then, a succession of Conservative ministers from David Cameron down have made political capital out of it. The roll-out to Swansea was even included in the 2015 manifesto.

So what has changed?

The Welsh Secretary Alun Cairns says the technology has, with diesel-powered bimodal trains that can be used on electrified and conventional lines.

No doubt the technology is improving but the developments have been well-known about. The trains were ordered by the government some time ago.

Ministers at Westminster are not talking about finances but the backdrop to this decision is money.

Last year, the National Audit Office said the estimated cost of modernising the Great Western mainline since 2013 had shot up by £2.1bn to £5.6bn.

If the finances are a factor, then the UK Government is going nowhere near it as an excuse.

I suspect it may well be that ministers at Westminster do not want to frame this debate in terms of money, particularly in light of the investment in HS2 from London to the Midlands and the north of England.

Instead the message is space and speed. The new trains will have bigger carriages and because of speed restrictions on the line between the two cities, there would be marginal differences in journey time.

And the argument goes that rather than facing a highly disruptive construction period, the new trains will be operational in the autumn.

Be that as it may, a broken pledge is a broken pledge, and the accusations of betrayal and broken promises have been flying thick and fast.

The Welsh Government wasted no time with the Economy Secretary Ken Skates coming out with his well-rehearsed claim that ministers in Cardiff invest proportionately more in rail infrastructure than Westminster, even though it is not a devolved a matter.

A Welsh Conservative source told me much of this response would have come as no surprise.

The challenge will be to make the case about a practical time-frame and hope that commuters do not get too hung up on whether their trains are powered by diesel or electric.