However, the friction of these bearing surfaces and their
location at a considerable distance from the center pin combined to
restrict the free movement of the truck. By the early 1850's the point
of bearing was transferred to the center plate, producing a truck that
turned more freely.[2]

[Illustration: FIGURE 2.--The 4-wheel Bissell truck as shown in the
drawing for British patent 1273, issued May 5, 1857.]

[Illustration:

A--Truck frame
B--Equalizing lever
C--Locomotive frame
D--Double incline plane
(_Centering device_)
E--Truck bolster
F--Swivel pin
(_Pivot point_)
_Drawn by J. H. White. June, 1960_

FIGURE 3.--Typical 4-wheel Bissell Safety truck of 1860. This drawing is
based on plate 69 of Alexander L. Holley's, _American and European
Railway Practice in the Economical Generation of Steam_, New York, 1861.
(_Smithsonian photo 46946_)]

For single axle engines this simple form of truck was entirely
satisfactory, but it proved less satisfactory for 4- and 6-coupled
machines. Also, as train speeds increased, so did the number of
derailments. Many of these could be traced to the inability of the
engine to negotiate curves at speed. Levi Bissell, a New York inventor
who investigated this problem in the 1850's, correctly analyzed the
difficulty. He observed that when the engine was proceeding on straight
tracks the leading truck tended to oscillate and chatter about the
center pin, and he noted that it was this action that imparted a fearful
pitching motion to the locomotive at speed. The derailments were traced
to the action of the truck as the engine entered a curve.

This action can be more easily understood from reference to Bissell's
patent drawing in figure 2. For example, let us say that an 8-wheel
engine, fitted with a center-swing truck, enters a right-hand curve. The
left truck wheels bear hard against the left rail. The drivers jam
obliquely across the track, with the right front and left rear wheels
grinding into the rails. As a result, the locomotive tends to leave the
track in the direction of the arrow shown on the figure (bottom
drawing). It will be noted that the truck center pintle is in fact the
fulcrum for this leverage. Under such strain the truck wheels are
particularly likely to leave the rails when they encounter an
obstruction. Once derailed, the truck would then spin around on the
deadly center pin, throwing the locomotive over.

In effect, then,