That doesn't make any sense -- witness the fact that an Acela trainset is 12000 HP, comparable to large freights, and is run on the line with many other trains at the same time.

Put another way, the resistance of cm-diameter copper, which is typical for overhead lines, is about .09 ohms/mile. For 50 mile spacing of substations, the average train is 12 miles from the nearest one, which gives a line impedance of 1 ohm. Let's make the train really big and claim it is 25k HP (~5 diesel engines). That pulls 700 amps over a 25 kV line, dissipating 490 kW in the line, about a 3% efficiency hit, which is not a problem at all. The efficiency hit increases as the square of the train power, so you hit a wall if you want to pull much more than ~80K HP per 20 miles of track -- but that's a lot and you can increase it by stringing some extra cables. I suspect the study you were referring to used a lower voltage?

That doesn't, of course, address whether the ROI makes sense in any case since the capital cost to install wires and substations is very large and diesel is quite cheap.