Linelayers are specialised spacecraft that are used to transport wormhole mouths to distant locations. When in transit a wormhole mouth is very small, only a few picometres across, but may weigh a trillion tonnes. For this reason linelayers must be very powerful vessels. Early linelayers used conversion drive systems but in the Current Era many linelayers have reactionless drives.
Modern linelayers are capable of relativistic speeds, which means that the wormhole mouths can become separated temporally as well as spatially. Because of this temporal displacement a wormhole or nexus of wormholes could, in theory, form a closed timelike curve. In practice any wormhole that could potentially result in a closed timelike curve will collapse due to the Visser effect.
This problem can be managed in two ways - firstly by separating the wormhole mouths by a safe distance to that they are outside each other's light cones (see this page Wormholes and Causality for an explanation of this method). Note that all connecting wormholes in a nexus must observe this condition.
Secondly in the modern era a pair of wormhole mouths will usually be sent off in linelayer ships at similar velocities but in different directions, so that both mouths become temporally displaced to a similar extent and the effects cancel out or are greatly reduced.
Linelayers either carry a scalar field generator to inflate the wormhole in the target system after arrival, or build one on site from local materials. These devices take in conventional matter/energy and use it for power to generate the exotic matter/energy needed to expand the wormhole.
Linelayers tend to travel with a top speed of around 0.74c. To reduce temporal displacement effects, most travel at around 0.707c, the speed at which the local frame of reference of the ship (and wormhole) experiences one standard year for every light-year travelled. On occasion void-ship linelayers have been reported, which can apparently travel much faster. Exactly how the massive wormhole mouth is incorporated into the voidship bubble is not known.