Yes. The problem is you do not know what you are doing. You need to use the right tool for the job. I'm not sure what a "curtain rail" is, uinless it is like a curtain rod, but what should work very well is an angle grinder with a metal cut-off disk, which obviously you do not have. I go after anything metal with my angle grinder and it works beautifully, and I can cut through just about anything, including solid steel.

It's very difficult to cut through thin metal with a hacksaw blade because the teeth on the blade are spaced farther apart than the thickness of the metal. In any situation like this the teeth then get hung up on each side of the metal and jam the blade.

If this thing is a tubular thin metal hollow rod, you cannot use a plumbers pipe cutter because the curtain rod has a seam down one side and is too weak to hold up to something like that. It would simply collapse.

Now, you might be able to take a wooden dowel stick the same size as the inside diameter of the rod, insert this into it for support, and then score around the outside of the rod with a utility knife by rolling it on a flat surface. If it is a flat curtain rod this method will not work. You might be able to use a flat metal file to file through the rod instead.