There have been electric delivery trucks for a long time - for example, Smith Electric Vehicles has been making li-ion trucks almost as long as Tesla has been around. And they follow up on a long history of electric delivery vehicles on a continuous line dating back to the early lead-acid days. But "existing" doesn't mean "having blown the market wide open". The big question is when that could happen.
You know, though, as ridiculous as it sounds, I almost wonder Tesla's efforts could evolve into a killer delivery vehicle. The Model S / Model X drivetrain is already starting to get into the power range of a big rig, and big rig budgets can afford their high prices. Combine that this potential solution to charging over long distances and you really could have a winner.
So the primary issue for Big Rig trucking is distance.
Drivers often fill up 100-200 gallon tanks, and cover large distances between stops. They might stop for fuel once a day, may be twice while covering 500-1000 miles. Can we build a big rig that has the torque capacity AND the distance requirements? May be.
But then, you also need to be able to manage the batteries well enough too, and be able to recharge quickly.
Battery swapping won't really work. Why? Who owns the batteries? Who's responsible for them? What places are going to swap out batteries?
If the owner of the vehicle owns the batteries, then swapping doesn't work. Someone that just bought the batteries isn't going to swap them for a set of used batteries - that's too expensive.
A network of battery renters could work, but then you have to be able to get to one of their approved locations for a battery swap, so it's not really feasible, though this solution could bring the cost of the big rigs down considerably since they wouldn't have to buy the extremely expensive battery packs with the big rig, just sign up to a renter network.
Even then, who's responsible for batteries when there are problems? Who pays the bill when the battery overheats and blows up? Or who sends out another vehicle to swap batteries when the drain too quickly and the vehicle is stranded?
Yes, lawyers can solve many of the questions, but until that happens in a manner acceptable to the industry then there are problems.
Also, realize that many big rigs are owned by their drivers. Independent truckers are quite common and many trucking companies prefer to use independent truckers over buying their own equipment; often paying barely enough for the guys to be able to maintain the vehicles - it's a very cut throat business. So those buying the big rigs don't necessary have budgets for expensive technology; they just want something that is going to work, work well, be easy to maintain, and be highly reliable - it's very expensive for them to get a tow (loss work, plus the cost of tow and maintenance). So it'll be a very up-hill battle to get them to accept electric vehicles.
For truckers to accept electric trucks, companies will have to get into it first. So FedEx proving that this vendor is reliable with their fleet will go a long way to bringing hybrids to trucking at all levels.
As to the transition of the technology from trains - the really big difference is that for the trains it's generally easy for them to know exactly how much fuel they need for the trip and how much to have in reserve. The fuel tanks are in the thousands of gallons, and they don't have to deal with traffic much. When they do, they'll know how long the wait will generally be and can handle the situation accordingly. It's also relatively easy for them to just fill up the tanks again. Just saying - the two industries are extremely different in many respects; so what one finds acceptable won't necessarily be so for the other.