attached to the main frame served another important function : they 

 allowed the locomotive to vibrate laterally between the running 

 rails. If the gripper wheels had been rigidly attached to the running 

 gear it w^ould have put an undue strain on the center rail. 



On the level, at stations or fueling places, where the weight of the 

 train would not be available to help start it, a steam cylinder or 

 heavy spring could apply the necessary pressure on the gripper 

 wheels. The steam cylinder could also apply more pressure if the 

 wheels should slip while pulling a light train up a grade. The weight 

 of the train could also be used to advantage when descending a 

 grade, in this case as a brake. The toggles were made double so 

 that they could press the adhesion wheels against the center rail 

 whether the train was ascending or descending the grade. In this 

 way the auxiliary cylinders of the gripper wheels, working forward 

 or in reverse under light steam, could hold the train back on heavy 

 gradients. The center rail, it should be noted, was not to be laid 

 the entire length of the line, but only at heavy grades, stations, and 

 other places where extra traction was needed. 



It was claimed that since adhesion was achieved by the pressure 

 of the gripper wheels rather than by the weight on the drivers alone, 

 a 10- or 1 2 -ton engine on the center-rail plan would produce the 

 same tractive effort as a 20-ton machine of the ordinary pattern. 

 This was the chief advantage claimed for the scheme. The same 

 results could be achieved by rack or cable, but Sellers was quick to 

 point out the complexity and expense of such systems. A more 

 detailed accounting of the principles and claims of this invention 



Figure 27.— Details of the Center Rail (fig. 2), adhesion 

 machinery (figs. 3 and 4), and safety clamp brake for the center 

 rail (figs. 10 and 11) as shown in the 1847 British patent. In 

 fig. 4 of this plate, "P" is the steam cylinder used to increase 

 pressure should the draw-bar weight prove insufficient; "KKK" 

 is a coil spring to keep the lever engaged ; "HH" are the massive 

 levers connected to the frame by pins "J-J" which support the 

 adhesion wheels; "I-I" are the toggles. "A" is the center rail, 

 "B" the adhesion wheels, "L" the draw-bar which connects the 

 train and toggles, "M" a pin by which the draw-bar could be 

 held stationary. The distortion of "B" in figure 4 is due to the 

 drawing lying across the fold of a page. 



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