Stmtrolleyguy--
Evidently the people who BUY passenger locomotives in the U.S. haven't been convinced that we need six-axle (or even five-axle, like the FL-9 or FM's passenger C-liner) units!
I tend to harp on the issue in posts to this forum, but I'm an amateur, a railfan, with no technical training: there's no particular reason anybody should listen to me. But, in case anybody IS interested, my reasoning is along these lines:
---The 268,000 pound P40 is far from the heaviest four-axle passenger diesel in recent years: a number of commuter lines have bought significantly heavier units.
---There's a real difference in weight PER AXLE: the E-8 was maybe abut 55,000 pounds, some recent four-axle power closer to 70,000: a 27% increase.
---Particularly at high speeds (so maybe this would be more of an issue for Amtrak, which is looking at sustained 110mph running on, e.g., their Detroit-Chicago line), higher axle loads lead to more track damage. (This may be less of an issue for commuter railroads, where the top speed is usually lower.)
---You're right that having idler axles and only four motors loses you tractive effort, but American passenger trains aren't limited by tractive effort concerns: at high speeds, the locomotive's power can be fully utilized with only four motors. (A commuter operator, with more stop-and-start, might be more interested in low-speed performance, and so worry more about tractive effort.)
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And, of course, on a GE forum it's worth noting a configuration that might combine the best of both types: A1A trucks, but with pneumatically operated levers to transfer weight from one axle to another, as in the ES44C4. (Motive for the ES44C4 seems -- from what press releases said -- to be more a matter of cost reduction, by replacing two expensive traction motors by a cheaper weight-transfer mechanism, rather than avoiding track damage, but…)
A unit like this could, at low speeds (where tractive effort is more of a concern and where the "dynamic augment" leading to track damage with heavily loaded axles less of one), transfer more of its weight to the powered axles, and then lighten them when operating at speeds where track damage is a worry and the available power can be utilized with lower weight-on-drivers.
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