FALSE, the heaviest Item in Minecraft is a Banner created using Notch Apples, which results in a Mojang Logo. And since you can craft the Logo onto the Banner multiple times, it would be even though being invisible, much more heavy than a single Notch Apple.
Meta-items not stacking in minecarts is probably because they implement IFluidContainerItem and Railcraft by default prevents you stacking liquid containers in chest carts (presumably so you'll use tank carts).
This is entirely on Railcraft's side, and I would have reported an issue, except that apparently I'm banned from the Railcraft repository on GitHub. I didn't even know you could ban people from repositories, and I didn't even know that repository existed until now (so I can't possibly have done anything with it to get banned).
Let's see if I can get someone else to make a PR for me.