I imagine weight is the main problem. A 400 hp electric motor is one heavy mamma jamma. I know some of these new GE & GM locomotives weigh over 400,000 lbs. Then you get into braking issues. Are they going to use dynamic brakes to replace the loss of compression? And if that works you have to retrain a bunch of drivers. Sounds like a good idea if they can work out all the kinks. It will be like all other innovation. If it can be produced and operated economically it will be produced.

Well, these new locos are at or about 6000 HP. So you have a 6-axle Loco, that's 1000 HP per axle from the Electric Motor. if you put a whole Truck's work of electric motors in where your 5th wheel is, that's 3000 HP coming from your old 500 HP Cummins and DD Engines/now Generators... Electrics are so efficient anymore, you just might pull it off. Dynamic Braking aside, i don't think it'd be needed, since the Efficiency of the Diesel-Electric type Tractor covers for it. You'd have to have the engine in 8th (on a locomotive, 8th is the final gearing in the Electric Motor, a.k.a. FULL BLAST) to really have a large in-efficiency, and even then, Locomotives have ever really gotten up to about 110 MPH (which is about as fast as your current tractors)

Now take your original estimate of weight: 400,000 lbs. Divide that by the 6 axles # 6000 HP and you get about 66666.66-Repeating (about 66666 and 2/3 lbs Per Axle) of power that is supported. 1 Axle is enough to satisfy a lot of loads, assuming a tractor is 20,000 lbs. 2 Axles of that power will pull everything you can think of, hell, could even pull a Swift Driver out of a ditch! 66666 and 2/3 x2 (axles beneath 5th wheel) = 133333 and 1/3 lbs that can be pulled, just by assuming the Locomotive's weight alone, not how much train behind that it's pulling. So you could realistically run 1 Axle Neutral 99% of the time and run the other axle plainly. Then if one gets weak, switch to the other one. Production on it can be economical, Call GE or GM, they make tons of em every year. The only barrier I see is fitting a Generator Assembly behind a standard Semi-truck engine, and training drivers to drive Electrics.