The robot transporter is a large machine with two silver 'shells'. These 'shells' also appear to have yellow, black and red stripes, and are divided by three columns of electricity.
There is a dilapidated version of the machine placed in the Present of the Rounds. It looks similar to the regular one, but all the colors are replaced by shades of blue and there's no electricity going through it.
Sonic the Hedgehog CD
Although its story purpose is debatable, the robot transporter provides a very definite purpose in Sonic the Hedgehog CD. In gameplay, it is placed in the Past of every first and second Round. Robot transporters exist in the Present as well, but are inactive and indestructible, merely giving a clue for where they were placed. Destroying a robot transporter in one Zone will grant a Good Future for it and will destroy all Badniks there. Destroying all the robot transporters in the game will earn the player the Good Ending for Sonic CD. Alternatively, they can also collect all the Time Stones to destroy the robot teleporters, and so achieve the Good Ending for the game.
In Sonic Mania, and its expansion Sonic Mania Plus, a robot transporter makes a cameo at the end of Stardust Speedway Zone Act 1, where it is destroyed by Sonic, Tails and Knuckles before they use a "Future" Time Warp sign to travel to the Present version of the Zone. Later, in Sonic Mania Plus' Encore Mode, the group, along with Mighty and Ray, would destroy another robot transporter in Stardust Speedway Zone Act 1.
↑Christian Whitehead (December 14, 2011). Sonic the Hedgehog CD(2011). IOS. Sega. Instruction Manual: "To ensure a good future for a Zone, you must either collect all the Time Stones, or destroy Eggman’s robot transporter contained within the Past."
↑Christian Whitehead (December 14, 2011). Sonic the Hedgehog CD (2011). PSN. Sega. "Savior of the Planet Achievement: Destroy all the robot teleporters and Metal Sonic holograms in the past."