Website with this circuit: Click Here
This is the circuit outlined on the site. In my case I did not have a complete HO scale train set so I didn't have the power pack.
This part I used as shown. The diodes cause a progressive drop in voltage with each block section to slow the shuttle as it approaches each end.
What I did have was a 12 volt plug in transformer from some gadget I tossed out. (I always save the power supplies) so This circuit uses a adjustable voltage regulator to provide the throttle.