This is true for many modern high-speed trains (though those tend to not be diesel trains...), but those are only a small fraction of all trains.
Upgrading all existing freight cars with dedicated motors would be a crazy effort. Of course you could argue that freight trains aren't modern, which would make your comment correct in the worst sort of way ;-)
This may be the case on electric passenger rail, and generally is the case for high speed passenger rail, where each carriage, and sometimes each wheelset, is powered.
However even for electric freights, you'll typically have a small number of traction wheels on the actual traction units.
http://en.wikipedia.org/wiki/Multiple-unit_train_control covers the gist of it. One of the challenges of long trains is that you can't easily start them on a curve. As the front of the train is pulled it wants to 'straighten out' and that would pull mid-train cars off the track, Such trains if they don't use MU put locomotives both in the back and the front (and some times mid-train).
The advantage of standardized MU is that your engine is simply a control center (especially if you're using power from overhead catenary wires) and any train of any length can "work" because the traction motors in every car are sufficient to push that car with some flexibility (some can push up to two additional 'dead load' cars.)
But as others have pointed out, in the US the tracks are primarily straight, the existing traction motors in diesels system works well enough, and there is little incentive to change. That said, one of the options on the table for electrified CalTrain is MU equipped cars so that train sizes can be varied depending on commute load.
MU allows you to have multiple locomotives controlled together, but individual train cars still generally don't have traction motors. There are specialized traincars that have motors but not generators [1] but I don't think these were what foobarian was referring to.
The trains in the u.s. are still pretty primitive. Do a google image search on amtrak for some cringeworthy pics.
Where I live in Europe the only trains where the engine pulls are freight trains. All passenger trains except some cog-wheel trains in the mountains have diesel electric generators and individual motors on each wheel.
While Switzerland has opted to go all-EMU for its passenger trains, that's not true of every country.
For example, the only powered axles on the French TGV are at the front and rear of the train. All the axles in-between are unpowered. The TGV isn't exactly a primitive train.
The TGV was the basis of the Spanish AVE and the Korean KTX. The KTX also uses additional powered axles on the first and last cars, but it still has a lot more unpowered axles than powered ones.
As for Amtrak, the Acela isn't a primitive train. It's a mashup of the French TGV and the Canadian LRC. And yes, it has unpowered axles in the middle. Its main problems are its excessive weight (forced by FRA regulations), and the totally inadequate investment in the track over the last 80 years. The basic design isn't that bad.
Well, the AVE 103 is based on the Velaro platform which most certainly has powered axles on every car, as you would expect from a German train. The same goes for the Russian Sapsan, the Chinese CRH3 and the German ICE3
In the USA, freight is the vast majority of rail traffic, excepting a few highly populated corridors in the northeast and commuter lines in a handful of other urban areas.
If you think propulsion is primitive, try braking. I was surprised to learn recently that braking is typically transmitted from car to car by compressed air, so that it can take up to 2 minutes for the braking instruction to reach the last car in the train.
Just as it's tricky for hte locotomotive to start the whole train at once, if the air braking system should fail, the momentum of the cars is likely to exceed the braking power of the engine: http://www.retronaut.com/wp-content/uploads/2013/01/317.jpg
"As a result of this and other runaway incidents involving locomotives with dynamic braking, the Federal Railroad Administration reversed its mandate that dynamic braking be disabled when train brakes are placed in emergency. The mandate now is that they must all remain functional."
What was the original rationale behind the emergency brakes disabling the dynamic braking?
I know, I just wanted an excuse to post the Montparnasse photograph again. There was actually an air-brake related derailment in Canada just last January on a train carrying crude oil, though luckily without major loss of life this time. I was surprised to learn that electronically-activated braking systems aren't the norm.
