It requires infinite energy to travel at light speed in ordinary space.
The faster you go the more your mass increases the more energy you need to travel faster... Oh and time seems to be slowing down the faster you go, so to not go more slower the faster you go, you have to pile yet more planets into your boiler.
Warp bubbles simulate a different version of space where the physics are different inside the bubble.
You actually have to go to a different dimension to travel at transwarp.
That has me suspect that a transwarp corridor is the opposite of a warp bubble. It's really a real space tube. Creating an area of real space inside transwarp space where we can exist without bad transwarpy things happening.