In the game a <a href=http://mtm2.com/~mtmg/traxx/cps.html>checkpoint</a> is an invisible box that triggers when the truck passes through it <i>in the proper direction</i>, in the track editor it is nothing more than an object box set to "checkpoint" (any object can be used as one) and oriented properly.
<center><img src=http://mtm2.com/~mtmg/traxx/images/cp01.jpg></center>
The checkpoint boxes used in the stock tracks (pictured above) are often used in custom tracks. Available in two fixed widths, they are black rectangles with white arrows on them. The arrows are downward pointing on the sides and forward pointing on the top and bottom, to make checkpoint orientation easier. These checkpoint markers do not fit all situations and in my opinion they should <i>never</i> be used. Instead, track makers should use "object boxes" as checkpoints. Inspect any track by an experienced track maker and you're very likely to see object boxes used as checkpoints. An object box is a non-model purple box, pictured below.
<center><img src=http://mtm2.com/~trackville/pics/cpob.gif></center>
There are many good reasons for using these resizable boxes as checkpoints, but the main ones are that they'll fit any situation and never require tilting to fit the terrain.
In Traxx, in Object mode, just right-click and select insert to create an object box (aka "collison box"), then double-click it and adjust it's properties.
Here is a trick for proper sizing and orienting an object box checkpoint: (note the above image)
Place the visible checkpoint markers in your track, using a banner or flags or whatever, at any distance apart. Then, no matter which direction your checkpoints may need to face to match your markers, face north and then insert a box in front of you (facing north like this should eliminate any orientation confusion). The side of the box you are now looking at is the side the truck must pass through to trigger the checkpoint (i.e. the southern side of a default object box is the 'active' or 'front' side). Checkpoints, whether made of models or object boxes, have a fixed 'directional orientation', with a front, a back and two sides, and they must be driven through from front to back in order to register.
Next, change the middle "size" value to something small like "1", as depicted above. This is the depth of the checkpoint box and a value of five or less is best (the stock ckbox bins are too fat in my opinion). You should now have a true rectangle, which is easier to visualize in use as a checkpoint as well as keep track of during rotation.
Now you can change the first "size" value, which is the width of the checkpoint (96 units or 3 terrain squares in the above example). A simple width equivilency chart is included in the above image, but the beauty of resizable checkpoints is that you can place your checkpoint markers ANY distance apart and resize the box to fit. One grid square is 32 units (feet) wide.
Next you can rotate the checkpoint to match your checkpoint markers, bearing in mind that you want to face the 'active' or 'front' side toward the oncoming traffic in your track. You can rotate it by changing the "orientation" value in the properties box, or better yet just select the box and press "R" to rotate it. You can also rotate a selected model by holding the shift key down while you move the mouse (thanks Phin).
Finally, you can drag the checkpoint into it's final position and make any necessary size and angle adjustments (object boxes are often positioned best when using an overhead view). Make the object box as tall as you like (20 in the above example) and then sink it into the terrain in order to eliminate any possible gaps due to changing terrain height.
Now you can delete the sorry 'ckbox' bins and never look back.
+'s:
+ Not using the ckbox models makes for one less bin in the track and one less 64x64 texture in your pod.
+ Fit any width or height.
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