I think you two are talking about two sides of the same coin - you're just trying to use different arguments about the same thing.

You're actually both right.

But diesel electric engines are used on trains because of the better torque curve of the electric motors. On hills and from a standstill, with trains with a couple of million tons of load, they just would not be able to pull a load well if they had to try to work off gearing down from a direct shaft from a diesel motor. There's just not enough low end torque to get a decent sized construct (locomotive/car combinations) moving. That's part of the reason the switch from steam engines to diesel-electric happened so fast. Steam locomotives were very limited in their total load capacity (usually only about 15-20 cars max) because of their poor ability to get going from a stop. Diesel-electric constructs can easily run into 100 or more car combinations nowdays.

The limitations they have been running into in the last couple of decades on the length of the constructs are not the power of the locomotives but the actual length of the string of cars itself. When they have to slow down to go through towns, a long string of cars tends to piss off the townspeople when they have to wait at the crossing for 15 minutes to let the train go by.