I don't know the exact contractual arrangements that Irish Rail have with the supplier, but one would imagine there is a technical standard somewhere that says a train should only sway X amount, lurch Y amount and wobble Z amount. One would hope that such a standard was included in the contract and that Irish Rail would hold them to it. It seems silly that it has goen on so long.
Its probably speculation to say its down to any one thing. I suspect its a combination of the train and the track, much of which is 25 years old and thankfully, they are starting to replace it, with a heavier rail being used. The wobble is much less on the new track.
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