Thanks for the reply. For what it's worth if you're ever in the US, Amtrak does offer very spacious seating and often enough you'll have your part of the row to yourself. Probably comparable to flying first class, I'd say.
Yes American trains are great, I've only used regional trains from New York but they were spacious and comfortable. Your Miami - Jacksonville pairing is interesting, I was thinking about it and I think the big difference between the US and the UK is that if you couldn't take the train in the UK all those people would be on the road and it would cause a traffic meltdown. Whereas in the US most people are going to either drive or fly between regional cities like that and the whole country is criss-crossed by freeways and flights that link cities together in a big mesh so you can drive or fly quite directly to your destination. I get the impression that most freeways outside of big cities have relatively predictable journey times in traffic. You can fly Glasgow/Edinburgh - London but it is also a busy route and there is no capacity in the London airports for more regional flights. Likewise pretty much all North-South car journeys in Britain are on two motorways which are very busy. The drive will currently take anywhere between 7-10+ hours depending on what time of day you travel.
