It's not a matter of how many engines you put into the consist. You're thinking about how much motive power or horsepower you need to move X number of cars. Instead it's a matter of how much electricity can be generated by the diesel motor to turn the traction motors at a high rate of speed. Just because you add extra engine(s) doesn't mean you can force the traction motors to turn faster. The only thing that can force the traction motor to run faster is more electricity.
And therein lies the problem. There is a maximum amount of electricity that a diesel motor can produce at a given size. The only way to get more electricity is to put in a bigger motor. Put in a bigger motor and something has to give, either you take something else out of the body of the engine or you make it longer, which now means you start to limit yourself to longer radius curves. You also start to get into a weight issue here, as to how much weight are you placing on the rails and the bridges, as you increase the size of the diesel motor.
Even if that's the case, what's to stop you from removing one catenary locomotive that has two trucks with traction motors, and replacing it with three diesel locomotives, each with a prime mover, with the middle diesel having no traction motors at all, and the other two locomotives each having traction motors on one of their two trucks only, with power cables running between the locomotives much as slugs use on freight trains? You should end up with the same amount of power on the powered trucks with the three diesels that way that you would get with the catenary locomotive.
This is probably something that PRR60 could answer better and with far greater certainty than I can, but I believe what stops you or at least makes it much harder, is the need to synchronize the power output. You can't just plug two generators in series without issue, much less three. The big power failure on the NEC a few years back was caused by this very problem. Amtrak wanted power generation from a small plant in Philly to supplement their power needs. When they tried to add that power plant to the grid, something wasn't right. So instead of providing power, the generator actually ended up drawing power off the NEC.
This caused other plants to shut down just to protect themselves from the overload being caused and led to the big blackout. But again, the bottom line here is that it's not easy to take power from one engine and combine it with another.
And then on top of that, you want to add all that extra weight and take away traction motors. Each engine currently has 4, and you want to drop it down to 2, while adding all that extra weight of a third engine. Frankly I'm not sure if you'd actually be able to move the train under those conditions. Remember the actual area of the steel wheel that touches the steel rail is probably about the size of a dime. Maybe on level ground on dry rail you might not have slippage, but I'm betting that going uphill or trying to move on wet rail, the engines would just spin their wheels.
The bottom line is that if all the engines in the consist are designed for a maximum speed of 110 MPH like the P42's, then whether you have one or twenty engines in the consist, your maximum speed is going to be 110 MPH regardless of how many cars you are pulling.
If the traction motors are designed to have a top speed of 110 MPH, then yes, that's going to be the speed limit no matter how many of them you have. But that by itself doesn't demonstrate that you can't put a 200 MPH traction motor and truck on a diesel any more than the catenary locomotives Amtrak uses for the Northeast Regional that are limited to 125 MPH demonstrate that the TGV doesn't reach 186 MPH.
I wasn’t suggesting that you can’t put a traction motor into an engine that can’t go faster than 110 MPH. We already know that Acela’s traction motors can safely do 150 MPH, and IIRC under extreme conditions can hit 167 MPH. However there is little point in putting in a 200 MPH traction motor if you can’t supply enough power to turn the traction motor at 200 MPH.
The point of the above paragraph however was really about the idea that by adding extra locomotives, or essentially overpowering the train, that it would be able to go faster. The above paragraph was directed at that. Put another way, if it requires one P42 to move 5 coaches at 110 MPH, adding two more P42’s isn’t going to make the train move any faster than 110 MPH. And that is what you seemed to be suggesting, hence my reply.
