Welcome to the forum!
The service manual does not show precisely the placement of the toggle spring. It does however reference it to the shift lever, so it may possibly be used to keep slight tension on the shift lever to keep it from shifting in motion due to vibration.
Perhaps clear pictures of the frame underside with the shift lever removed, and a picture of the shift lever and toggle spring can be posted if you still need help in determining the position.

If all else fails, a call to Lionel's service department may help, as they have a Postwar Celebration Series remake #6-18464 that seems to employ the same design as the postwar unit. Lionel does not have any service information available for that unit, unfortunately.

Larry

Larry