I don't know what physical issues exist but there are immense challenges with these projects, not least of which is that you often have to do most or all of the

I don't know what physical issues exist but there are immense challenges with these projects, not least of which is that you often have to do most or all of the work before you can get any benefit. You then have old trains that need replacing as well, driver training, maintenance training etc. Oh, and you also need to find a workforce of several thousand skilled workers and mostly do the work at night so that you can still run a train service. In the UK, they recently electrified a lot of the Great Western mainline and decided to go with a cheaper/quieter type of installation method - auger drills iirc - for the pillars (why not), but at certain points, they started finding large granite boulders from the original railway construction which wouldn't be drilled, which meant new plans, new equipment, new training and loads of cap-ex when they were still years away from service. They also found tonnes of electrical system conflicts which meant whole signalling systems being replaced even before they had life-expired. I love trains and I love modernisation but most of us underestimate the enormous amount of work involved in an upgrade like this.