Most horizontal-launch systems envisioned in the past, that used electromagnetics as a launch-assist, were subsonic. In order to conserve fuel during launch and to keep engine-weight to a minimal a combination of electromagnetic-propulsion, i.e. maglev or rail-guns and scramjet engines may offer a viable alternative for manned systems to LEO. A lot of electrical-energy storage systems would be required to create the required velocity in the launch infrastructure such that the orbital vehicle scramjet engines could ignite. An example of a lower speed system is here. If the launch velocity could be increased to between Mach-2 and Mach-4 a manned system, using only scramjet engines, could be constructed. Final orbit insertion would need some additional rocket engines. The launch-track would need to be at least 10 miles in length to keep acceleration under 3 g's, and offer launch-abort opportunities.
The potential of a horizontal launch within the earth's atmosphere is very much limited by the atmosphere itself. At a low height the maximum speed is limited by the high atmospheric pressure and at medium height there is not enough oxygen left for the scramjet. But to get into a low orbit, much more speed and height is necessary, about 8 km/s speed instead of some 1.3 km/s for Mach-4 and 200 km height instead of only about 25 km. The difference in kinetic energy is impressive, about 38 times more for 8 km/s instead of 1.3.