The span-bolster type four-truck wheel arrangement appears to have originated in the 1920s, where it was used for some interurban electric locomotives. Initially it was in articulated form, with the two span bolsters connected by an articulation joint that carried all buff and drag forces, the mainframe being total free of such.
The first “large” locomotive to have this B-B+B-B running gear was Illinois Central #9201, later #9202, of 1936, and built by the St. Louis Car Company subcontracting to EMC.
Kirkland (1) equated this type of B-B+B-B running gear with that of the NYC T class electrics, built from 1913. But the T class did not have span bolsters. Rather, the outer trucks acted as pilots to the inner pair of driving axles, which were rigidly attached to the main truck frame. One may think of it as having been a 2-B+B-2 type with powered pilot trucks, although in fact the T preceded the first of the latter wheel arrangement. Furthermore, the power pilot trucks of the T had no provision lateral for motion, and concomitantly, the inner axle pairs were mounted close together. Thus the whole assembly would have behaved somewhat like a 4-4-0 steam locomotive.
The GE GTEL4500 prototype appears to have been the first to use independent span bolsters, that is, not connected by an articulation joint. In part that may have been to improve the riding and tracking at higher speeds, and in part to allow space between the truck assemblies for underhung equipment. It also had lateral motion (swing-bolster) trucks, which were more suitable for higher speeds than the rigid-bolster types. That GE had a “horses-for-courses” approach is evidenced by the more-or-less contemporary VGN EL-2b class motor-generator electric, which had articulated span bolsters and rigid-bolster trucks, suited to heavy pulling at low and moderate speeds.
The mounting of the GTEL4500 couplers on the span bolsters might have been done to minimize throwover on tight bends. As noted upthread, this meant that all buff and drag forces went through the span bolster pivots. That requirement of truck pivots had been seen previously, for example on Illinois Central prototypes #9200 and #9202 (later #9201), of 1935-36. These had independent C trucks (i.e. they were C-C, not C+C) with the couplers mounted at the truck outer ends. Also, GE had used truck-mounted couplers on the 1939 steam turbine-electric prototype pair tested on the UP. These had the 2-C-C-2 wheel arrangement, with unconnected main trucks, so that the main truck pivots handled the buff and drag loads. This wheel arrangement was actually quite rare (estimated worldwide total 12), whereas the more familiar articulated version, 2-C+C-2, was much more common (estimated worldwide total 495).
N&W used the independent span-bolster running gear for its 1954 steam turbine-electric prototype #2300, in this case with C trucks. Here again, the couplers were mounted on the span bolster ends. Brown Boveri, Switzerland, proposed several GTELs in the late 1940s, one of which had used span bolsters with a mix of C and B trucks.
Given that UP repeated the GTEL4500 span-bolster running gear in the 1960s for the Alco C-855 and GE U50 types, I think it would be reasonable to infer that it had satisfactory riding and tracking characteristics at UP’s operating speeds.
(1) John F. Kirkland; Dawn of the Diesel Age; Interurbans Special 80; Interurban Press, 1983; ISBN 0-916374-52-1; see page 185.
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