Using the Camera Rig Rail - lock position on Rail to Player Movment

I’m trying to bind the current position on a camera rail (0-1) to the movement of the player character.

The player can walk in 3D space. But the camera rail is only in 1 direction (like a sideways dolly/tracking shot)
I want to have the camera base follow along with the players movement as the player walks past the camera deck. Getting closer or farther away from the rail would not effect the cameras position on the rail, but walking along the rail, the camera would follow in translation. I already have the camera set to track the Character blueprint and focus set to track the character as well. So aiming at the player is all taken care of. But I still need the Dolly to follow the players position on the track. (I’ll most likely want to set up the trigger volume as a volume that the camera tracks while in the volume, and stops when the player leaves the volume [on end overlap])