I keep getting pushback from Amtrak defenders about my article about its locomotive order. I think I addressed most points, but one that I didn’t that keeps coming up is whether electric multiple units are really better for train service than locomotives hauling unpowered cars. The answer is in Amtrak’s case an unambiguous yes, but it requires more argument.

Ordinarily, the cost tradeoff between multiple units and locomotives is that unpowered cars are less expensive and lower-maintenance than EMUs while locomotives are much more expensive and higher-maintenance. EMUs have definite advantage in performance; they accelerate faster, and, when the consists are short their energy consumption is much lower, since most modern locomotives are optimized for longer freight trains. Because the advantage is the most pronounced for short consists, Amtrak asked Vermont to buy US Railcar’s FRA-compliant DMUs for the Vermonter train, replacing the current diesel loco-hauled setup; Vermont itself puts the breakeven point between DMUs and locos at 4-5 cars, but the DMUs in question have just one vendor and are extraordinarily expensive by global standards.

Conversely, locomotives require much more track maintenance than EMUs, because of their higher axle load. Road wear is proportional to the fourth power of axle load, so the less even the weight distribution is, the higher the road wear is. Track wear does not satisfy such a neat formula; all old comments of mine stating the contrary should be ignored. However, for freight traffic such a formula does hold, and locomotives have axle loads comparable to those of freight trains. One could also observe that in Japan, railroads make every effort to keep axle load low, and therefore avoid articulated bogies; furthermore, almost all Shinkansen axles are powered to keep weight distribution even, whereas European high-speed EMUs only power about half the axles (Siemens’ Velaro has a maximum axle load of 17 t, and an average load of 14 t).

Generally, the trend in countries with well-run passenger rail systems is away from locomotives and toward EMUs. The exceptions come from three cases:

1. Some technologies, most notably the Talgo tilting wheels, can’t be used with powered bogies. The same is true of the tilting TGV test train.

2. Some railroads ignore track maintenance costs and focus on train maintenance. This includes SNCF, since the tracks are the responsibility of RFF.

3. Cultural inertia may make railroads too used to separate power cars. This again includes SNCF, which needed power cars for the TGV because of the technological limitations of the 1970s and 80s, requiring very large transformers.

In the specific case of Amtrak and the Northeast Corridor, not only are reasons 1-2 not an issue, but also the cost question favors EMUs. Look again at Vermont’s report, which seriously posits unpowered coaches costing up to $5.5 million each, more than a standard off-the-shelf EuroSprinter loco; Amtrak’s recent order is much cheaper, at $2.2 million per car, but still comparable to the FRA-compliant M7 EMU and not much less per meter of car length (and more per car) than the Coradia Nordic EMUs used in Sweden or the FLIRTs used in Finland.

In comments elsewhere, I’ve heard that one reason to keep the locomotives is that they can be detached and replaced with diesels on through-trains to unelectrified territory. This is pure cultural inertia; EMUs, and even power cars that are permanently coupled to unpowered coaches, can be attached to a diesel locomotive, as the TGV did to reach Sables d’Olonne. More cynically, the cost of Amtrak’s locomotives is $466 million, which, at Northeast Corridor electrification cost (about $3 million/km), could electrify 155 km of route, almost all the way from Washington to Richmond. At the cost of electrifying the line to Sables d’Olonne (about $1.2 million/km), it could electrify nearly 400 km. Amtrak’s insistence on locomotives is reducing flexibility here rather than increasing it.

But in general, the move toward EMUs is not about flexibility; railroads around the world deprecate it and have semi-permanently coupled trains. It comes from the fact that, outside Amtrak’s uniquely bad experience with Metroliner EMUs, they work better. I’ve already mentioned higher acceleration. In addition, all else being equal, they’re more flexible, and can be scaled to any length: the M7s are married pairs. I’ve seen commenters that claim the exact opposite, by looking only at EMUs with articulated bogies; those have nothing to do with the question at hand (the TGV has articulated bogies, too), and indicate that the operator cares about other things more than about flexible length, for example a walk-through train or reducing the number of bogies.

Another problem with locomotives, besides inferior performance, is limited capacity. A single-deck 200-meter long AGV has 466-510 seats, compared with about 350 for a single-deck TGV and 545 for a double-deck TGV. SNCF is still eschewing the AGV because its capacity limit is so great it needs double-deck trains, but Alstom is developing a train with standard, unarticulated bogies that it claims can reach 600 seats with one deck.

Although Amtrak does not have the capacity problems of the LGV Sud-Est, it too is capacity-constrained, in another way. The limiting factor to Amtrak’s capacity is the lack of cars; as a result, buying EMUs instead of locomotives and coaches would add more capacity per dollar spent. It’s brutal, but true. Even the slightly more expensive Nordic EMUs would be an improvement; they’re still cheaper than coaches plus a single locomotive for all train lengths up to 14 cars (if the loco is an Amtrak Cities Sprinter) or 9 cars (if it’s a TRAXX or Prima).

In reality, the reason Amtrak uses locomotives is entirely cultural inertia. It was burned with the Metroliners, and thinks that unpowered cars last longer because, well, they have to. The reality that the M7, or the average European EMU, lasts 40 years, the same as Amtrak’s coaches; however, that idea was not invented by Amtrak, and is therefore out. It thinks that unpowered coaches are cheaper, while buying coaches that cost the same as EMUs. And so on. This is yet another bad US rail practice, hindering rail revival by making it too expensive and reducing performance.