question

A custom task sequence is probably the way to go with this problem, either with writing code or using Process Flow tools.

The thing that is a little tricky about this is that in order for the operator to appear to shadow the flowitem, they actually need to be heading to the same place at the same speed. In order to do a Travel task, the operator needs to know what the final destination is.

So I took your model and added a couple of decision points:

linea-with-shadowing.fsm

These will mark the "window" during which this operator can follow the car. When the car hits the first decision point, the operator will start walking alongside the conveyor and when they get to the second decision point the operator will go back for next car. In order to make this look right, I also set the operator and conveyor to the same speeds.

Next I created a Blank Fixed Resource Process Flow from the toolbar and attach the first decision point to it:

This is the environment where I will implement the task sequence to tell my operator to travel to that second decision point. the nice thing about doing this as a FixedResource (or instanced) Process Flow is that multiple objects can be attached, allowing those objects to take advantage of this logic. It also allows the use of 'current' and port referencing to gain access to the attached object and any objects connected to it via ports

In the Process Flow logic, I create an event triggered source that will create a Token when the car triggers the OnArrival event of the attached decision point, and I capture the triggering flowitem into a label called 'Car'.

Next I use a Custom Code activity that uses the 'Stop/Resume item on Conveyor' picklist option. This stops the car at the decision point in case the operator isn't ready to shadow the car. I assumed this is acceptable since other wise you'd have cars going down the line with no operator to follow it.

After the car is stopped, I use an Assign Label activity to figure out how car the operator needs to travel. I create a label called X_Distance (since the conveyor is set up along the X-axis of the model, I just need to know the distance between the operators origin and destination). For the value assigned to the label, I can type

current.outObjects[1].location.x - current.location.x

to find the total distance to travel along the X axis between the origin and the destination decision points.

Next I do an Acquire on a Resource that references whatever is connected to the origin decision points center port connection:

If the Operator is successfully Acquired, the car is restarted on the conveyor, and I give the operator a 'Travel to Loc' activity that tells the operator to walk the distance along the line:

Notice the 'Relative' check box is checked because rather than give an exact coordinate, I'd rather tell the operator to move a relative distance. This makes the process flow activities more portable to other pairs of decision points like these. Once the travel is done I send the operator back to the origin and release him for the next car.