The problem is, the railgun has plenty of cons (expensive to fire, needs a smaller gun to protect it) and the two pros aren't that useful.
The damage is absurd, but given that almost all of that damage is overkill, it's more effective to have a turret that has lower DPS overall but distributes it over more shots. Such as a regular turret filled with uranium bullets. Or a whole bunch of them considering that them and their ammo are far cheaper than railguns.
The range is good, but not enough to make it useful for covering a wide area. It only covers the width of about two small turrets, and since you need a small turret to protect it you're not giving yourself more coverage with less turrets (even ignoring that railguns are more expensive than starter turrets).

If railgun turrets traded off half their damage for cheaper ammo which stacks up to 200, and 60-65 range instead of 50, then I'd consider them a decent alternative.