The Cosmic Treadmill depends on the user's speed, thus its primarily used by the the Flashes (Wally, Barry etc...). The treadmill is advanced for its time but will eventually be outdone by the Time Bubble which allows anyone to use it. In Legion of 3 Worlds the multiple Brainiacs devised that special platforms attached to the Cosmic Treadmill would allow the speedster to take passengers through time and space.
In the new 52 continuity, the treadmill has been redesigned. In the new Flash series, Barry Allen is struggling to learn to control his powers as he embarks on his superhero career. When Barry runs too fast he creates wormholes which draw in matter and energy. Darwin Elias advises Barry he can help him and part of that is the creation of a new treadmill which siphons off the excess energy which Barry creates. However, at one point Barry uses it to create a wormhole so that he can go to find Iris West. The new treadmill is not purposefully designed for time travel.