Coupling Rods - Connecting the driving wheels together.
Declaring and calculating the basic values:

#local Gauge_ = 1.435; // standard gauche #local Wheel_R = 1.00/2; // wheel radius #local Con_R = 0.20; // conjunction radius #local Conjunction_Len = 1.20; // conjunction length = axis distance #local Time = clock + 0.00; #local Wheel_Angle = 360*1*Time; // rotation angel #local Wheel_Circ = 2*pi*Wheel_R ;// wheel circumference #local Wheel_Move = Wheel_Circ*Wheel_Angle/360; // Conjunction position relative to wheel center: #local Con_Y = Con_R * sin( radians( Wheel_Angle )); #local Con_X = Con_R * cos( radians( Wheel_Angle ));

Wheels and conjunction rod, we can use from macros defined in my railway system 'RW_System' According rails can be found in the rail track system 'RT_System'.

RW_Wheel_11

RW_Axix_11

RW_Coupling_Rod_00

RT_Track_Straight_00

2 Axes with wheels and concjunction rod:

//--------------------------------------------------------------// #include "RW_System/RW_Axis_11.inc" #local Axis = object{ RW_Axis_11( 2*Wheel_R, // wheel diameter, // in meter 12, // Number_Of_Spokes, // 0.20, // Connector Radius // 0 or >wheel diameter/2 // must be between: < 0 and < 0.5 wheel diameter 0.15, // counterweight depth, // 0=non, < wheel radius Gauge_, // gauge in meter ) //------------------------------------------// rotate<0,0,Wheel_Angle> } //------------------------------------------------------// #include "RW_System/RW_Coupling_Rod_00.inc" #declare Coupling_Rod = union{ object{ RW_Coupling_Rod_00( Conjunction_Len,// coupling rod length x+(link to link) 0.12, // rod height in y 0.06, // depth in -z // 0.09, // Rod_End_R // radius end links, >Rod_H/2 ! 1, // Oil_Fillers_On // 0 = non, 1= On ) //----- scale <-1,1,1≶// mirrowed to x- } // coupling rod axes cylinder{<0,0,-0.05>,<0,0,0.05>,0.03 texture{ Rod_Metal_1} } cylinder{<0,0,-0.05>,<0,0,0.05>,0.03 texture{ Rod_Metal_1} translate<-Conjunction_Len,0,0> } } // end union Coupling_Rod ------------------------------------// //--------------------------------------------------------------// #include "RT_System/RT_Track_Straight_00.inc" // #declare Rail_Spikes_On = 1; // optional, default = 0, #declare Track = object{ RT_Track_Straight_00( Wheel_Circ , // track length in x+ in meter 2.00 , // number of ties per meter; ~1.5 ) //---------------------------------------- } //------------------------------------------------------// //--------------------------------------------------------------// union{ // ---- 2 axes with connecting rods: object{ Axis translate<-Conjunction_Len,0,0>} object{ Axis translate< 0,0,0>} object{ Coupling_Rod translate<+Con_X,+Con_Y,-Gauge_/2-0.12>} object{ Coupling_Rod scale<1,1,-1> translate<-Con_X,-Con_Y,+Gauge_/2+0.12>} translate<0,Wheel_R,0> translate<-Wheel_Move,0,0> // move it by clock to the right! translate<0,Height_Rail_Top,0> // lift it to rail top } // end of union ----------------------------------------------// union{ // ----- adding the rail tracks: #local Nr = -3; // start #local EndNr = 4; // end #while (Nr < EndNr) object{ Track translate< Nr*Wheel_Circ,0,0 >} #local Nr = Nr + 1; // next Nr #end // --------------- end of loop } // end of union ----------------------------------------------// //--------------------------------------------------------------// // animated 'camera': #declare Camera_Position = Camera_Position+<-Wheel_Move,0,0>; #declare Camera_Look_At = Camera_Look_At +<-Wheel_Move,0,0>; //----------------------------------------- camera{ location Camera_Position right x*image_width/image_height angle Camera_Angle look_at Camera_Look_At } //------------------------------------------------------// //--------------------------------------------------------------//

....

Attention : We need to have the include files in the according subdirectories 'RT_System' and 'RT_System' somewhere in our include file path, as mentioned in the scene text above!
 
An example of an animation of this kind
you can find here: 3D-Animations - Railroad (Linked Axes).