Thankfully, train do not have checked luggage and no practical technological way to introduce it (i.e. something that does not require to rebuild all the stations, most of them deep in the cities) - so there is a huge commercial imperative to tone down the security theater .
I hope at least, I don't want to be proven wrong on that one. Boarding the Eurostar for example, is definitively not a 5 min job, more like 1 hour the bad days, but at least you can still carry most of your stuff and considering you do not have 1 hour transit to the airport, it still beats the plane by a good 2 hours.
Eurostar is quite exceptional. They recommend 30 minutes.
There's a passport check, and a 30km undersea tunnel, which is probably considered a terrorist target. Knives, alcohol etc are permitted, but not compressed gas or liquid fuel. (I tried this once, on a camping trip.)
You can take an almost identical train from King's Cross to Glasgow, arriving 5 minutes before departure (or less, 2 is the minimum recommended time).
I hope at least, I don't want to be proven wrong on that one. Boarding the Eurostar for example, is definitively not a 5 min job, more like 1 hour the bad days, but at least you can still carry most of your stuff and considering you do not have 1 hour transit to the airport, it still beats the plane by a good 2 hours.