PONZ, Stu hit the nail on the head. Lionel designed a truck with two mounting screw holes depending what car it would be mounted on....Paul
Paul - only one of the two trucks has the optional hole.
Ponz
PONZ,
Based on your photo, they both have the extra mounting hole. Based on the underframe, this looks like a Lionel Standard O Double Door Box Car. For this style car, the trucks should be mounted using the mounting post on the truck that's on the tab which extends very close to the inboard axle.
In your photo, the truck on the left is properly mounted (using the "optional" hole, as you call it). In the photo, the truck on the right also has this optional mounting post. I can see the tip of the post peaking out just behind the inboard axle.
Here's how to make it right:
Step 1, remove the 4 screws from the 4 corners of the black underframe to separate the frame from the body/shell.
Step 2, remove the screw which attaches the truck to the frame for the truck on the right in your photo. This is the one where the truck is attached through the hole in the truck bolster instead of the mounting post on the tab that sticks out.
Step 3, reattach the truck, but this time screw the truck mounting screw into the mounting post on the tab near the inboard axle (not the screw hole that's dead center in the truck)
Step 4, reattached the frame to the car
This is about a 4 minute repair and only involves unscrewing things and rescrewing them back to gether.
Stu