The Old World did reach a level of modern technology. What remains in the wasteland is mostly scrap, but can be put together to be useful.

That being said, there are vehicles in the wasteland, but they're basically a hodgepodge of other vehicles. Kinda of like death traps. In the sense of a actual tank, I think that might be a bit over doing it, but a heavily modified car would work fine (Think Mad Max)! Though these cares are very rare. The most stable mode of transportation are animals, and the Eden Trains to the gardens. Tanks and other military level vehicles would of belong to Eden, so they would go to great lengths to recover the scraps to get it working again. 

Metal is a good conduit for ruins, but you need to be an enchanter to activate it. Enchanters have a certain external energy that helps trigger the rune's magical potential. The spoken bit of it simply instructs the rune what to do.  

Eden is also all about restoring and preserving the old world. 

For an Engineering character, it's absolutely possible! Scraping together what they can in the Wastelands to survive. Using and modifying old world technology would be a really fun character to have! Kind of like a junk rat.