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Get some wire mesh
Fenris Games has a variety of fences and railings: Link.Plastruct makes O scale handrail which is a littler big for 28mm but looks fine: link.
Remember O Scale is smaller than "O Gauge", 1/48 is often used as a vehicle scale for 28mm whereas I think only TWD:AOW uses O Gauge (1/43) vehicles.
That is correct; I meant to type O Gauge.
Get some wire mesh (about 5mm squares). I bought one piece for a couple of quid 3 years ago& I still have about half of it left. Snip off what you want & apply. For railings leave spikes that will go into the model.If the 'model' is soft (e.g. foam board) push it into place & secure with the appropriate glue.if the 'model' is hard (e.g. plastic kit) drill some holes, put the 'spikes' in & glue.The mesh is also good for making ladders to go on the side of buildings, just leave 'spikes'on either side of the rungs, bend them 90 degrees & 'push' ( or place into drilled holes) andyou have a useful way that figures can get from one level to another.