A towel rail mounted to a fixed shower panel can be a neat solution where wall space is limited. It is not, however, an accessory that can be added after the glass is installed. The panel, holes, load, door movement and wet position all need to be considered together.
The panel must be specified for the hardware
The rail adds weight and leverage to the glass. The selected hardware must be compatible with the glass thickness, hole positions and panel size. The holes and cut-outs must be made before toughening.
Put the towel outside the spray
A towel rail inside the direct shower spray will make the towel wet and add unnecessary moisture to the glass and hardware. The position should be convenient to reach without becoming part of the wet zone.
Check the door and handle
If the rail is on a fixed panel beside a door, it must not obstruct the handle, seal or door swing. The towel should also be removable without standing in the shower or reaching across a moving door.
Consider the wall-mounted alternative
A heated or wall-mounted towel rail may be more serviceable and easier to replace. The glass-mounted option is strongest when it solves a specific layout problem and the enclosure has been designed around it from the start.
Read about why glass cut-outs must be planned before toughening, or see bathroom electrical planning for heated towel rails.