Fuel costs are significant
Saying that fuel is about 20% of the operating costs for any major railway in North America is a reasonable statement, ..since they are major users of diesel fuel, largely for their locomotives. So in respect of climate change and moving away from fossil fuels to renewables, ..here is a large target for making significant progress. ..It is a soft target.
The locomotives, especially the road units that move long trains long distances, are all diesel electrics that have electric motors which turn the wheels while the diesel engine powers the alternator that provides the electricity.
Pretty obviously, some other type of engine could instead power the alternator — say an external combustion [EC] engine preferably, since it can use biofuels entirely, displacing fossil fuel use by that locomotive.
This can progress in stages. ..Freight trains these days are very long in North America, often over three kilometres and increasingly over four. ..Pulling all those railcars are usually three locomotives at the head, assisted by another pair or three locomotives farther back. ..How many depends on the size of them, and locomotives keep getting more powerful: _a common size for the newest ones is 4400 Hp [3280 kW].
Now, while they all may be running at maximum output in order to start the train rolling and get it up to speed, not as much power is needed to keep the train rolling at the speed chosen for travel: _’cruising’ speed, so to speak [though rail workers likely use some other term].
In this state, none of the locomotives need be near its maximum output, though at least one in each group may be.. should that be wanted.
Opportunity for change
Here is where a turbine locomotive may be employed, one whose working fluid as hot vapour powers the turbine and then is condensed and sent back to the boiler for another cycle. ..This is known as a closed loop system, since the working fluid just keeps going round and round: _heated to a hot vapour, thru the turbine, cooled to a liquid, and then back to the boiler.
Such a turbine engine when operating near its design maximum output is about as heat efficient as a diesel engine: something like 40%, ..which is better than an automobile’s gasoline engine. ..The snag with the turbine is its distinct drop in efficiency when running at lower levels of output than its design capacity.
But as just mentioned, as one locomotive in each three pulling a long train long distances can be near its maximum output nearly all the time, why not use a turbine unit with external combustion? _And since it can use a variety of renewable fuels exclusively, colour this engine green.
<< One example of a cheap biofuel is solid pellets of annual plants, ..such as made from stalks of maize or canola or sugarcane bagassee. ..Less convenient than a liquid fuel, but then the ash — to the extent it has potassium carbonate in it — has value as a fertiliser. ..And if pulverised, the solid particles could be held in suspension by a liquid fuel, making it convenient to handle.>>
More change
That was locomotives on the road, the type with rails. But railways also have much smaller locomotives used for shunting railcars around railyards, and going short distances to and from a railyard. ..These engines are infrequently operating at their maximum output and so are unlikely to have a turbine engine, as that would mean storing excess power in some manner while running efficiently near full output whenever the engine is working.
But while pondering that, we might also recall a childhood friend: ..Thomas the Tank Engine. ..This type of locomotive carries its fuel and water itself and has no tender. ..This limits its range, before needing to restock. …But it has the advantage of being able to run both forward or backward at speed, (which a locomotive pulling a tender car typically cannot, as the tender isn’t stable being pushed).
But suppose instead of a boiler, it had only an insulated tank of very hot water that was released as needed to become the steam that turned its wheels. ..Or better, some other working fluid, with the need to condense same. _So then: ..a tank of hot fluid and a condenser tank that absorbs spent vapour and keeps it cool.
[Alternatively, perhaps cylinders storing high pressure air to turn the wheels and be vented after. ..Simpler design, though recharging with more air might be rather frequent. ..Which could be done using a small turbine/air compressor unit which would either be fully on or fully off, charging the several storage cylinders, one being charged while another is supplying compressed air to the air motor on the driven wheels axle.
That might not be feasible, too little power. ..But such a locomotive would cost considerably less than a battery electric one which is the other obvious alternative for a yard unit that does not use a fossil fuel.]
Later change
Having some turbine locomotives on the road is a start; ..changing the other road units to non~fossil fuels would be the next step. ..One possibility is a locomotive whose turbine has multiple vapour nozzles which can be cut in or out, so the range of output that is heat efficient becomes broader. .Or a different type of engine that is not a turbine. All of which is a different discussion for another day.