Energy efficiency isn't measured in terms of "Pound for pound of fuel." It is a ration of productive energy out vs. the energy in. A utility scale coal powered steam generator is only
about 37% efficient (a smaller sized generator would likely be less efficient). By comparison, a diesel-electric locomotive transfers
about 30-35 percent of the energy generated by combustion to the wheels (including the loss of the traction motor). When you take into account the reduced efficiency resulting from a smaller steam generator with the additional loss from the traction motor, a steam-electric locomotive wouldn't be any more efficient and it would be a lot more polluting. Now maybe a combined cycle gas powered generator in a locomotive
might be more efficient, but that would bring on its own set of issues.
I was trying to simplify it for those that may not fully understand it.
So, I was talking about 3 different cycles.
The Rankine, Diesel and Brayton cycles, all of which are theoretical, but are generally accepted when talking about efficiency.
Then we have the "Pound for pound of fuel." that I said. Most people do not understand what kj/kg are.
But really, we are talking how much energy from a given method is the cheapest per watt. This is where it gets messy as the math would show that on board energy conversion from the 3 cycles; Rankine, Diesel and Brayton that Rankine, and Diesel are close, depending on fuel, and cost of fuel. The discussion was about a gas turbine running a train. That is the Brayton cycle. The calculations show that of the 3 cycles, per watt, the Brayton is the least efficient one of them all. So, when picking a heat engine, many things come into play. One is the efficiency. The other is the cost of the fuel. If we took a similarly rated Rankine, Diesel and Brayton plant, using the same fuel, then a Rankine or Diesel could be used. But, let's say we don't have access liquid fuels. Well, the only one is the Rankine. Nuclear plants, such as Darlington, Pickering and Bruce all are simple Rankine plants that use the heat given off from nuclear fission to heat the water going to a steam turbine. So, if we ever can get a fission plant to meet the required crash standards, and be small enough to fit on an existing engine, there would be no need for batteries, or other prime movers to move trains.
A combined cycle plant, as you suggest might be better, but now we are talking space. Typically, those plants of a same sized watt rating are bigger. That means that on your typical platform, you would need more of them to get the same pulling power. This now comes down to: what is the smallest sized plant to create the most electricity? That answer is Brayton, Diesel, then Rankine. Combined cycle would be bigger than the Rankine as it needs the gas turbine, plus all the reheaters and other ductwork to get as much usable energy out of the heat created.
So, when I hear talk of gas turbines coming back, I laugh. There are reasons they are no longer used in much else besides aircraft.
While many (most?) trains are electrically driven, not all are. For example, many DMUs (and some locomotives) are
diesel–hydraulic.
True. Fun fact, both the RDCs and the Nippon Sharyo DMUs are both that variant. Having said that, there is no reason an electric motor could not be swapped in for the diesel engine. There is no reason that if any lines that use these are electrified, that they cannot be converted. You could get motors with the right watt rating and the right voltage.
Throughout this, I am using the watt instead of kilowatt to simplify things. I don't use horsepower because then I would have to talk about the slug..... and that just gets messy.
