Heavy-duty railsThe heavy duty rails are compatible with all workstations. Recommended for large and heavy keyboards. Lockable Special ball bearings ensure smooth sliding under heavy load Maximum load up to 120 KGs Compatible with Buso Audio keyboard trays. If you're buying the heavy duty rails separately as an upgrade to your existing Buso Audio desk, you'll need to choose the upgrade kit option: "Heavy duty upgrade for existing Buso workstation" as in this case we