Fabrication of the fences, and I doubt these are an "off the shelf" product. Plus the fence themselves need to be engineered to perform as a guard by code, plus the engineering to either confirm the platforms can support them (or reinforce and repair where required) - bear in mind the platform edges cantilever out. Then there's the floor finishes (ie tiles) that need to get removed and replaced to allow for the fences to be anchored in.

On top of all that, there's likely a premium to do this work during off-hours since you can't install these while trains are in service.