Swap motors between 5 locos and test each one multiple times? Interesting perhaps, but impractical in terms of time.
You can't remove the worms from these gearboxes without breaking the locos down to individual pieces. Again impractical.
None of the locos have any spring tension on the trailing trucks: they rely on the weight of the trucks themselves to keep them on the track, and it works fine in that they never derail. The leading trucks *are* all spring loaded, but only enough to keep them on the track; I.E. less than 1 oz each: an inconsiderable percentage of the loco's total weight on the drivers. (Roughly 26 oz.)
Southern Pacific GS-Class steam locomotives were all large 4-8-4s, and carrying the factory weights these all balance right between the center two pairs of drivers. Right where you want the CG to be located. The Balboa GS-2 that was poured full of lead balances slightly forward of that, but not as far forward as the second pair of drivers, and not enough to cause the front springs to sag measurably.
While I could -and probably will- easily test each loco's tractive effort for comparison purposes, it's the large speed differential between mechanically identical units that interests me the most.
I'm wondering if I could get consistent speeds out of them by switching to modern can motors -which would mean I could run any of them together while double-heading- but since they all run just fine right now I hesitate to swap motors only to find that they *still* run at different speeds!