Wikipedia introduces the idea of a Rocket Sled Launch (also catapult launch, ramped launch, etc) as follows:
With this concept the launch vehicle is supported by an eastward pointing rail or maglev track that goes up the side of a mountain while an externally applied force is used to accelerate the launch vehicle to a given velocity. Using an externally applied force for the initial acceleration reduces the propellant the launch vehicle needs to carry to reach orbit. This allows the launch vehicle to carry a larger payload and reduces the cost of getting to orbit. When the amount of velocity added to the launch vehicle by the ground accelerator becomes great enough, single-stage-to-orbit flight with a reusable launch vehicle becomes possible.
The idea has been used in fiction several times, but so far as I know, no one is even discussing building one. In these days of SpaceX and competitors, I'd expect that someone would be talking about it if it were feasible, so I assume that it's not currently.
Even if a direct launch to orbit from the sled isn't feasible, I'd think that this would be a efficient way to get a regular rocket into the upper atmosphere at significant velocity, instead of starting from a standing start on the ground. The rocket would then be able to push the payload the rest of the way into orbit with far less fuel than a traditional one would need.
What obstacles would a system like this have to overcome to be considered? Is it a matter of location? Materials engineering? Simply financial? Have there even been feasibility studies?