While it is theoretically possible, it is most likely impractical.
Specific concerns include the bowshock, and the resultant sonic boom. If using also a tube, the air itself will also generate a considerable force profile and tube-exit turbulent flow.
To use a non-evacuated tube, you're needing to generate supersonic airflow - this can be done, but requires a massive amount of moving air from high pressure reservoirs. (Functionally, any rifle does exactly this - generating the supersonic gas wave by detonation of gunpowder.) The effects of the rush of supersonic, or for launch to orbit, hypersonic, air out of the barrel will create a massive exit turbulence, and by venturi effect, drag surface air into the flow. It will also be the loudest cannon yet. To generate this supersonic flow, air will have to be added to the tube to prevent back pressure; at that point, the maglev may as well overpressure and make use of the pressurization, by massive pressure additions behind the vehicle timed to hit just as it passes. If the air in the tube is already moving at high speed (certain wind tunnels can produce over 400 kph), the in-tube drag can be reduced, but there will be a sudden deceleration when the non-moving air is encountered upon exit.
Note that supersonic flight, especially in the hypersonic region, is a high drag configuration.
The pressure wave from the barrel exit will, as previously noted, transfer a lot of kinetic energy to the exterior air, resulting in an upward flow, which creates a surface low, which draws air in from around. This will pull surface materials towards the exit point. This is a multiple hazard situation - it's a hazard to anyone or anything near the exit point, and it's a hazard for repeat use because the exit port must be kept clear of debris.
The bowshock only exists when the projectile is faster than the medium. All objects moving through fluids generate bowshock - but it's only a significant noise situation when the speed is a high fraction of the speed of sound in that fluid, relative to the fluid itself. (A bullet doing 400 m/s down a 200 m/s airflow isn't generating a 400 m/s bowshock, but a 200 m/s one.) Since the bowshock is a major portion of aerodynamic and hydrodynamic drag, the bowshock is going to vary widely based upon the nature of the air being moved through.
The bowshock will sound like an explosion (and is, mechanically, nearly identical to one at range), as the transition to supersonic speed happens. The air, since it cannot accelerate quickly enough through normal kinetic transfer, converts the movement energy from directional vector to vibration. We hear vibrations, and this generates the large sound.
In an enclosed tube, the boom becomes relevant upon exit (the tube itself must be constructed to survive it. In an open system, the entire path of the rail must be capable of withstanding the noise and outward air rush. This makes the unenclosed supersonic system a major environmental quality issue for noise pollution, in addition to the previously mentioned venturi effect and ground drag.
Further, the post-vehicle shockwave can generate a low pressure zone that itself causes a second boom - few aircraft hit velocities high enough for the human ear to notice this second, weaker, cavitation, but in water, cavitation causes bubble formation and demonstrates the principle - the noise of a high speed submarine is both the bow shock and the collapse of the partial vacuum behind it. A launch system will have speeds that generate a much longer cavitation event than self-propelled aircraft do. This is going to be perceived as a longer boom, or even a second boom (certain high powered rifles generate a separate cavitation sonic event).
So, in the end, there is the turbulent airflow, the disruption of surrounding landscape, and the noise pollution, plus much increased drag.