Interventions By The Others

The Avatar. 

The Others do more than provide interstellar transport machines for species that achieve interplanetary capability.

They also:

directed humanity to the Phoebean System;

gave the Danaans metal tools to use on their gas giant planet;

gave the pulsar dwellers the Oracle;

constructed observation stations for other races at the pulsar and at a black hole.

Also, the Oracle, a self-conscious and hyper-intelligent artifact, gives Chinook data that will enable the holothete to plot a probable path to the Others' frontier. Something like this was necessary. Random rotations around T machines would not have returned Chinook to the Solar System where a major problem awaited resolution. 

Another novel could describe a ship travelling indefinitely through space-time via T machines. Such a ship would require a self-sustaining internal environment and also some mechanism to ensure that each T machine led to another T machine, not just to empty space. With these requirements in place, the sky would be the limit. Or rather would not be. The Others travel between universes.