Why are locomotives replaced when successors little more powerful?

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I'm curious about something..as I peruse books on locomotives through the years, the text will say that a loco got to the end of its useful life. Often, the successor is not that much more powerful, measured by HP, and I wonder what the advantage of the new model is in the first place?

Is it not a good use of money to rebuild/replace worn out parts on existing locomotives, and cheaper to develop and purchase a new one?

Maybe there's more to this than just the few tech specs I'm reading, but if Loco A is 2400 HP, and is replaced by Loco B, also 2400 HP, what's the gain?

thank you!



New Member
Modern locomotives have several advantages over older models:
* better traction -- usually through the use of computer controlled slip detection which can reduce power when slip is detected, or redirect the power to other axles -- just like the anti-slip and ABS brake systems in cars these days.
* the use of alternators and AC motors for a complete AC drive train -- rather than alternator-rectifier-DC-motor or generator-DC-motor system. These allow for much higher continuous power output.
* more fuel efficient and/or output lower emissions
* tax benefits in buying and writing off a new locomotive. In the past capital rebuild programs also qualified for tax deductions but I don't know if this is still the case. In addition, a rebuilt locomotive has an extended life much less than the expected life of a new locomotive.

Another modern trend is the use of distributed power units (DPU) in trains; whereas in the past a string of 5 or more SD40-2's or similar could be seen at the head of a train, now you're more likely to see 2 units at the front and 1 or 2 units at the end of it. Newer locomotives can be purchased equipped for this while retrofitting existing units is often not economical.


New Member
You need to consider this anymore with anything that burns a fuel. The cost of the Fuel used to run it. Why well one Locomotive could get better SFC meaning it burns less fuel per HP Produced meaning it pulls harder for less fuel burned. Then you have the cost of Maintaining the thing. Yes the Trainmaster was a beast however when you have to disassemble half the engine to change a lower piston it kind of makes them shop time intensive compared to say an SD24. Then you have other concerns that enter into it like Parts problems or how they are running on the road. The RS 3 was a good locomotive just the Turbo on the engine was a POS and that cost them customers. There are a lot of reasons why locomotives are replaced by new ones that are not much more powerful anymore.

Right now the Elephants in the room are Fuel Costs and Emissions. A SD40-2 burns 200 GPH and makes 3000 HP. Or you can get a GEVO that makes 4400 HP and burns 250 GPH. Which one better for your bottom line. Then you have Emissions with the EPA.


Thank you for those excellent points. I had no clue about those GPH figures, but that is good to know.

Now, back in the day when fuel was less expensive than it is now, say, 30's up to maybe the 70's, was shop time the big expense? Or has fuel always been the #1 factor in deciding "useful life?"

Bill Anderson

Well-Known Member
I asked related questions regarding the lack of success of the 6,000 hp locomotives which were introduced in the late 90's(?). Has present technology reached a point where locomotive horsepower is going to peak at 4,000-4,500 for the foreseeable future? Will future R & D explore alternate (and less expensive) fuel sources rather than increases in horsepower?

As has been pointed out above, some of the smaller diesel locomotive manufacturers, like Fairbanks-Morse and Baldwin, failed to receive follow-up orders when their initial models proved unreliable and spent a lot of down time in the shops. Another factor was standardization of a railroad's diesel roster. It was much less expensive for a railroad to keep parts in stock and adopt maintenance procedures for a few standard models (which used interchangeable parts) from one or two manufacturers than for several one-off models from 4-5 different manufacturers.


5th Generation Texian
Part of the calculations depend on accounting rules; a certain % of the engine must be ‘new’ for it to count as new from a tax perspective, which as mentioned along with EPA rules are more of a driving force than almost any other factor. Unless the rebuild has a certain % of new content, it cannot be accounted as capital expense. Capital expense has a lower effective tax rate than other levels of rebuilding.

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