SD70Dude
Despite the extra horsepower, I believe a SD45 has the same starting tractive effort as an SD40.
Of course it does; they have the same traction motors that are just as heat-limited up to about 12mph, so the engine horsepower is almost irrelevant in that range. Arguably something with a 6-567 could develop equivalent starting TE if given the motors and adhesive weight and run up to notch 8 with a suitable alternator ...
it just would balance out awful quick...
So it can haul the same train, faster.
That was really the only point of the SD45; in fact, if you
couldn't make use of the higher speed, the locomotive was not as fuel-efficient. Note how many SD40-2s are still in service vs. the 20-cylinder engines; in fact there appears to be no service need for the 20-710 SD80MACs now and they've been put up for sale ... could be amusing to see who needs the unique combination of characteristics they have.
Also recall that the 3700 HP 251F as used in the LRC was not particularly reliable.
Perhaps even more to the point, nothing all the king's men could do could
make them reliable, which to me is a sure sign you're trying to extract more horsepower than the engine will deliver. Even operating a 251 at 1100rpm is likely "too much" -- and some regimens of operation called for 1200rpm, which to me would be science fiction on any suspension or trackwork that Alcos would be likely to operate over in the '70s.
Note that this is separate from the issues in operating a 251 with high boost and better steel-capped pistons but lower rpm, as in some of the current Indian practice, which apparently can get reasonable life at over 30psi intercooled boost and over 1800psi peak firing pressure.
I don't know what other engines were planned for the LRC, but it is unfortunate that Alco didn't survive as a United States locomotive builder long enough to obtain Valentas under their existing Ventura cross-licensing agreement. That would have been a better starting point for an LRC, and the logical "improvements" over the years (specifically the VP185) would have worked about as nicely for the LRC trains as they did for the HSTs.