Steve, I've got the old CN set of two with the common truck. Can't get to it now but I believe it came with another common truck so you could mate two or more sets together to make a 5 car unit train. I forget if there was also a different end platform to facilitate this. Check the Lionel website to see if you can get the instructions for the 6-16360.
As for parts I've had luck on ebay, search O scale trains for Maxi Stack. There is a regular vendor who has parts. The pictures make it easy to make sure it's the part you need.