Yes, there are urban outdoor furniture pieces that can be locked to a railing for security, but they are usually a mix of purpose-built products and smart aftermarket locking methods rather than one standard category.
The most direct options are railing-mounted or clamp-on pieces: folding seats, small café tables, planters, and bike racks designed to attach to balustrades or railings. Some of these use padlockable clamps, locking pins, or tamper-resistant bolts, so the furniture can be fixed in place and released only with a key. These are common in compact public spaces, parks, and commercial patios where floor anchoring is not possible.
Another practical route is to choose outdoor benches, chairs, or storage seats with welded eyelets, lock bars, or hidden security loops, then secure them to a sturdy railing with a hardened chain, cable, or U-lock. This works best as a deterrent against opportunistic theft. It is not high-security protection, because the railing itself and the lock become the weakest points.
For urban settings, look for suppliers that offer anti-theft street furniture, lockable railing mounts, padlockable clamps, and tamper-resistant fasteners. Also check local rules before attaching anything to public railings, and make sure the railing can safely carry the load. Avoid blocking walkways, emergency exits, or accessible routes, and consider removable cushions or lightweight components to reduce risk.
In short, yes—lockable railing-mounted benches, tables, bike racks, and clamp-on seating do exist, and standard outdoor furniture can often be secured to a railing with the right lock. Just match the security level to the location, and confirm that both the furniture and the railing are suitable for the job.