approx 6 h1/2 hours
2 days
About a day.
Amtrack can take up to 20 hours. It was 23 1/2 from Tampa to DC last year for me.
If you take a direct flight it is 7 1/2 to 8 hours. I know this from experience.
19 hours and 15 minutes on the City of New Orleans
The trip from Chicago to New York when riding the "20th Century Limited" took 16 hours in 1930. The travel time varied depending on which train was taken.
21 hours and 45 minutes on the Texas Eagle.
Around 3 hours, give or take 20 minutes. You can also take the Amtrack train straight from Portland to Tacoma, or Seattle. It takes generally the same amount of time, but I prefer the train, because it's more comfortable, and often cheaper than gas. I used to live in Gig Harbor, and now I live in Portland.
Unfortunately, there is no rail service to Nashville. The nearest major cities with Amtrak service are Memphis and Atlanta.
There is no direct train route to Milwaukee from Salt Lake City. You would have to take a 36 hour train ride from Salt Lake to Chicago, followed by a 2 hour train ride from Chicago to Milwaukee. If you left Salt Lake at 3 a.m. on Saturday, you wouldn't arrive in Milwaukee until Sunday evening... I would definitely not suggest taking a train from Salt Lake to Milwaukee, it would be cheaper and much faster to fly.
You can't travel directly from Amsterdam to London by ferry because London doesn't have a ferry port. You would have to travel from Amsterdam to Hook of Holland by train (or drive), catch a ferry to Harwich and then take a train (or drive) to London.