Rage doesn't keep you from mounting back up, it just restricts the circumstances in which you can do so.
The Rage rule states that you have to move as fast as possible towards the nearest visible enemy unit during the Movement phase, and is suspended while embarked. Hence, assuming at least one enemy unit is visible, it would not be legal to mount up at the start of your Movement phase or in the middle of your Movement phase, because by doing so you would voluntarily be not moving as fast as possible towards the nearest visible enemy unit. However, you could move as fast as possible towards the nearest visible enemy unit, and at the end of that move embark on a transport, as you would have satisfied the requirements of the Rage rule for that Movement phase.
I like to think of it like this: Rage units won't get on a transport, but that doesn't stop the transport from chasing them with a vacuum and sucking them in at the end of their move.