From TM Wiki

Trackmania United Forever

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Trackmania united forever was released as a free expansion to Trackmania United, Adding a couple of new blocks to each environment as well as new skins for the cars.

For example The island environment got the "purple" boost, wich gives more speed than the yellow, but less than the red. Also the Coast environment got bigger and longer "underground slopes".

Nadeo also updated the Manialinks, how much i do not know.

They also changed the menu layout to be easier to navigate.

The track editor now also has a simple interface not to scare away beginners.