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Perhaps this sounds like a daft question because most people would consider it to mean track, signalling, motive power and rolling stock, but read on...  ;D

On Thursday lase week, 17th June, I was on the 1445 Leeds to Kings Cross (an HST by the way) sitting with a couple of local government officers who were on their way to a meeting in London. The conversation turned to railways in general and funding in particular, which always appears to be concentrated in London and not the north (so they said and I'm just reporting and not taking sides!). The matter of HS3 was brought up, a proposed new line from Manchester to Leeds and, so they say, this has now been paused/ shelved/ abandoned in favour of improving the infrastructure on the existing lines in the area.

This got me thinking.

Whilst we all think of the term infrastructure to mean track, signalling, motive power and rolling stock, there is also the alignment of the railway to take into account. All railway routes across the Pennines involve going up one side and down the other. In the days when they were built, when the opposition was a cart horse or a canal if you were lucky, it didn't really matter much if the lines twisted and turned a bit to ease the gradients for what were, at the time, state of the art but still underpowered steam locomotives. Today, however, things have moved on but those curves and chicanes are still there.

Therefore, as I see it, you could electrify and shave a few minutes off with improved acceleration and reduced dwell times with automatic doors, but if the curvature is such that limits the line speed to, say, 40 mph over some sections, you could be using the most advanced electric stock known to mankind, but it still wouldn't be going any faster around those bends, or across that curve viaduct, than a 156, a 142 or anything Horwich Works was building in the 1890s for passenger work.

What do others think?

To my way of thinking 'infrastructure' refers only to what used to be called 'way and works'. It does not include rolling stock.

As you rightly point out, a curve built in the 1840s for a 40mph balancing speed would still have the same restriction today. Possibly not quite so severe as modern surveying methods allow for improved transition curves and cant change rates - maybe one can get round at 50mph today.
Straightening the alignment for higher speeds has a long history. In open, flattish countryside it is no big deal. It gets expensive if it needs land take, or the geography confines the possible alignments or buildings close to the lineside make purchase and demolition very expensive.

So it all comes down to the calculation of whether the additional income from faster services and/or reduced operating costs can be balanced with the cost of the works. Unless of course political influence can be brought to bear.