Description

There's more discussion of the origin of this in #3622, but the gist is that track files write multiple sets of particle states for a single track as a single vtkPolyLine object. This results in a segment appearing at the end of one set of particle states leading to the beginning of the next set of particle states. This segment doesn't represent transport, however, and can easily be misinterpreted as such when visualizing these tracks in a tool like ParaView.

There may be benefits to writing all sets of particle state in the same vtkPolyLine, but in the simplest case we could simply write each set of states to a new line.

Alternatives

If it's important that particle states from a single track be written to the same vtkPolyLine object then we could perhaps use segment IDs to indicate which of the segments move from the end of one state set to the next so they can be filtered out as needed in visualization tools.