Fortunately many critical infrastructure system and life safety systems have on-site backup energy generation capability, as long as diesel is available. At least in the USA, our internet, payment, and phone systems have all withstood significant medium-term regional power outages.
A major infrastructure cyberattack seems effective either as an opening salvo in a traditional military war or to multiply the chaos after a terrorist attack. Taking out the power grid would worsen traffic congestion and create increased demand on emergency services in the short term, and have major economic impacts and high visibility in the long term.
American here. Trains already don't run, no real net change.
If just 10% of a nationwide grid is down, there's a good chance the phone network won't work, internet will be down, trains won't run, credit card/payment systems won't work, etc. All those things have primary and backup systems, but somewhere in the chain of dependencies there will be both a primary and backup that have been shed, and the system designers thought "these two data centers are 500 miles apart, so won't fail together".