The trend is for larger and larger locomotives run in multiples with a single crew. For new locomotives what are the advantages of one large one vs two half that size?
See the discussions of the UP 'double diesels' for many of the arguments in brief. (Also, some of them go the other way, for why a B unit may be preferable in some circumstances to a cabbed locomotive...)
There were some discussions back in the late '60s and early '70s that, under some circumstances, the two 'half' power engines would burn less fuel to produce equivalent ton-miles. There are the old EMD arguments: if one of the half-powers goes into the shop or dies on the road, the other one may be able to help carry on.
A potentially big issue is that reliable railroad diesels appear to 'max out' before the 6000hp level. In part this is due to compromises in design needed to fit the prime mover into relatively cramped railroad clearances. The EMD 265H that was so carefully designed turned out to develop ultrasonic cavitation and block wear when driven to full power -- probably-unavoidable resonances and focused energy at little points in the block structure, rather than a more familiar 'critical speed' in the engine, but troublesome nonetheless.
In any case, a prime determinant of practical horsepower in a consist is siding length; another is yard approaches and capacity. At this point, cars can't really get any heavier (in important senses they're too heavy already!) so length determines required power much of the time. Multiples of 4400 seem to be convenient. Most railroads that tried modules of 6000 found the 'granularity' too coarse...
Now, for a different potential 'take' on this -- consider NREX 3600, with its two QSK50s -- like two 1800 hp locomotives on a common frame. Shut one engine off when power isn't needed, or run them both 'lazy' for intermediate power demand. (I also like the features of the QSK series...)
So far we only have reasons for two rather than one ( which I agree with). I was hopping someone would post reasons for just one
Lab So far we only have reasons for two rather than one ( which I agree with). I was hopping someone would post reasons for just one
If You're rolling along with 3 locomotive on the head of your train and one of the units dies, you can carry on with 2/3 the horsepower.
If someone builds a 13,500 HP locomotive and uses it as the sole power for a trains it better be the most reliable diesel electric ever built or there's going to be a lot of dead trains blocking the mainline..
One of the key advantages to diesel electrics is that they can be M.U'd together to reach the desired power so railfans who talk about "superdiesels" seem to miss that..
"I Often Dream of Trains"-From the Album of the Same Name by Robyn Hitchcock
There was an argument made for the Baldwin Centipede that it was physically shorter than any combination of 'units' of equivalent horsepower at the time. Fewer units = more loads in a siding length, for instance.
There are arguments for one diesel engine of higher power rather than two less-powerful ones: compare the complexity in, say, an E7 to an Alco PA of equivalent horsepower. Fewer ancillaries, shorter length.
I remember some vague rumblings about 6000 hp locomotives being able to take certain trains on their own, or in shorter consists, when those locomotives were being introduced. Part of the logic was that higher build quality, performance guarantees, etc. would assure one of these locomotives could take a train by itself under the right conditions. (Did not work out for a variety of reasons).
I expect there may be a revival of higher-horsepower single units ... but in genset form. (Thereby re-creating the original Baldwin 6000 HP single-unit design from the early '40s, now become economically practicable...) My own thinking is that these things would actually work as road-slug mothers, to give any combination of combustion power and number of TMs.
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