Why are American trains all 'stainless steel' looking in colour and design? Also, from what I've seen, even their new stock looks way behind what we have here. Has anyone travelled on trains in the USA, especially in the North East? If so, what are they like internally.
I have travelled quite a bit in the USA; multiple times during a gap year in Canada 2005-6 and then a handful of subsequent one-off trips on long-distance Amtrak trains. In fact, in the spring of 2006 (as late as I could possibly start my trip with low season winter pricing) I used the now-discontinued "North America Railpass" to build a 30 day round trip of the USA and Canada on Amtrak and VIA Rail. I wrote about it on Blogspot, and you can still read the blog (albeit in reverse chronological order) here:
http://jamesbrownontherails.blogspot.co.uk/
Broadly speaking, as others have explained, Amtrak have two core fleets... the Amfleet (for routes in the North-East, and out of New York City) and the Superliner (mostly out of Chicago). Superliners can't fit through the tunnels into Manhattan, hence the division. There are some other smaller fleets of single deck cars in the east (Viewliner ... these are mostly sleepers and diners), mid-west (Horizon) and north-west (Talgo) and some double-deck trains along Amtrak-operated commuter routes in California. As mentioned, Amtrak's paltry high-speed train is the Acela, along the Washington - Philadelphia - New York - Boston corridor. Acela may not compare well to European high speed trains, but it is relatively unusual for Amtrak in that it makes money

Note... Acela only has Business and First Class. If you want "coach" class (economy) you have to take a slower loco-hauled regional train.
I spent most of my travels (between Montréal and New York City, and between New York City and Chicago and Louisiana) on Amfleet cars. Amfleets were built in two generations, with a different number of doors and differently sized windows. But on the inside, like the coach class Superliners, they have big squishy cloth covered reclining seats. They are comparable in size and comfort to old fashioned (1990s era) North American airline business class. In the north-east there is also Business Class, which are moderately wider seats in a 2+1 formation.
Regarding the age of the fleet, Amtrak has a very unfortunate situation in that its constitution means it effectively needs to go cap in hand to the federal government for funding every single year. Long-term planning is, therefore, very difficult, and even the more liberal governments find it a struggle to invest heavily in passenger rail. There is an ongoing order for additional Viewliner cars to supplement long-distance services, mainly out of New York City, and the Amtrak Acela will have a new fleet in the next few years.
Internally they are completely different, it would take an article to describe. Seats always face forwards only, and the entire train is turned around at each terminal point. Separate toilets for men and for women at opposite ends of the car. One thing that has not spread from Britain is the seats have never been squeezed up to just pack more in, the spaciousness and seat comfort is still just as it always was.
Taunton makes a valid point - Americans hate going backwards on trains! But where train sets cannot be reversed using a Y at their terminus, the seating on both the Superliners and the Amfleets have a kick-lever beneath each pair of seats. It is time-consuming but possible for every pair of seats to be unlocked and spun around... or if you know how to do it yourself, to turn two pairs into a facing quad, albeit with very little leg room.
Meanwhile, in Canada, it's a whole different story. VIA Rail has a profitable and popular core "corridor" service from Windsor, Ontario (south of Detoit) through Toronto, Ottawa, Montréal as far as Québec City. This route is served by a combination of really old stainless steel carriages, 1970s era LRC (light, rapid, comfortable.... or
léger, rapide, confortable) carraiges and even some ex-BR Nightstar "Renaissance" carriages. The Renaissance cars also serve Montréal - Halifax.
VIA Rail's business is different from Amtrak... Amtrak really does compete with Greyhound bus and the low cost airlines for long distance travel across the country. But across Canada, VIA Rail's flagship "Canadian" (Vancouver - Toronto in about three days) is a big bucks tourist train. There are a handful of essential service trains to remote communities in northern Québec and Manitoba, but the latter is on borrowed time because of the likely permanent closure of the railway line from The Pas to Churchill, Manitoba.