The Humern empire uses Photonic Railways to transport its unimaginably vast cargo containers from one solar system to the next. These ships can reach truly staggering velocities (>0.8c even for 'short' journeys) before finally stopping their acceleration phase.
Each ship on the railway is a series of huge, very precisely crafted mirrors, and as such any form of impact could be catastrophic. The ships that form the 'train' follow one another along a very finely controlled series of lasers. Each ship reflects the light for their own propulsion back to the base station, and the light for the next ships further along the line. Needless to say the Humerns have some pretty impressive precision mirrors, laser technology and material engineering skills.
Even with all their technological prowess interstellar gases and debris can still cause an issue for the photonic railways, and so the Humerns use 'Ploughs'. A Plough is a large blast shield (usually 10x the diameter of the following ships and designed to be thick enough that it will survive the trip) and set of massive gyroscopes, designed to either absorb or deflect interstellar gas and debris away from its more fragile followers. It requires a lot of laser stations at the sending end to speed it up, and often the Ploughs are allowed to 'crash' into elliptic orbits around the target star for capture using more conventional means rather than being slowed by laser stations at the destination (or sometimes they're just flung into the interstellar void).
The question is this: How many Ploughs will be required to clear (and maintain) a path through the somewhat inaccurately named 'void'? Will a large fleet when the railway is established be enough to clear the path for generations to come, or will each train require its own Plough to clear the way?