one end of which was
attached to the locomotive under the smoke-box and the other to rear of
the truck frame, at the same point of attachment as in the Bissell plan.
Thus, according to Hudson, the pivot point could move laterally so that
the truck might more easily accommodate itself to a curve of any radius.
He further claimed that a better distribution of weight was effected and
that the use of the radius bar relieved the center bearing casting of
much of the strain of propelling the truck.

[Illustration: FIGURE 8.--A 2-wheel Bissell truck installed on the
Pennsylvania Railroad's _No. 91_. This engine originally an 0-8-0 Winans
Camel built in February 1854, was rebuilt by John P. Laird in 1867, at
which time the Bissell truck was added. Note that Hudson equalizing
lever was not used. (_Smithsonian photo 46806-k_)]

The British journal _Engineering_, in an article otherwise friendly to
the inventor, expressed some skepticism as to the real merit of Hudson's
invention.[15]

If Mr. Hudson's truck, ... be examined, it will be seen that the
radius link serves no other purpose than that of carrying the truck
along with the engine, and this could obviously be equally done by
the pivot or central pin of the truck itself.

It is probable that few builders other than Rogers made use of the
Hudson radial link.[16] One of these was John Headden, whose _General
Darcy_, shown in figure 6, was fitted with the Hudson truck.

Thus, by 1860 there had been perfected and adopted a successful 4-wheel
safety truck for 4-4-0's and 4-6-0's used in general mixed and passenger
service. But as the decade advanced, the need grew for heavy freight
engines that could be safely run at speed. Without a pilot truck, the
leading driving axle of the freight engine was generally overloaded.
While the application of a 4-wheel truck reduced this front-end overload
and permitted faster running it materially reduced the traction of the
drivers by bearing too great a portion of the total weight. This loss of
traction was of course highly undesirable and generally disqualified the
use of 4-wheel trucks for freight engines. What was needed was a truck
which would guide the 0-6-0's and 0-8-0's around curves and yet leave
the greater portion of the weight on the drivers. The 2-wheel, or pony,
truck met these requirements.[17]

[Illustration: FIGURE 9.--Running gear and truck designed by John L.
Whetstone, as shown in the drawing for U.S. p