is it supposed to do that when i'm in motion? just starting to move i understand but it constantly jerks. this always happens in cab control which is why i almost never use it.
		
		
	 
Yes, it is supposed to happen when moving. As you start to move you will pull all the cars tight, you'll run out the slack. But thanks to draft gear, springs in the couplers to reduce draft and buff forces, the cars will sort of stretch a little as the couplers pull out of the car slightly. This pulling action will be uneven though out the train tanks to many factors such as grade, car weight, and car rolling resistacne. This means while you might have 10.000 lbs of strain on your first coupler, you might have 10,100 lbs on the second, and 8,900 on the third. Now with the springs in the couplers they will pull the car ever so slightly faster at some points, causing the cars to get closer and push on the next. This results in an ever so slight back and fourth motion though the whole train on level track.
Now when you get into grades, there might be a point where the train is pushing you down a hill, trying to pull you down the hill. But these buff and draft forces remain. Thus you get these sounds.
To add, as Azervich said, there is a gab in the knuckle on the couplers. This can be about 1-2 inches of open space between the the couplers of 2 cars.
To truly avoid this, use British equipment. They use spring loaded buffers, and tightly puled links to keep the draft and buff forces low.
Think of knuckle couplers as a big long chain. There is slop between the links but it moves when pulled and pushed. And British equipment more like a big pool noodle. Kinda squishy, flexible, yet solid.
As for your other question, regarding up grading. I do belive you can import content form other versions. But its a "do at your own risk" sort of thing.