There are many very large cities, New York, London as example, who must face the same challenges.  It would seem that a pooling resources to find cost effective solutions would have merit.  
The other risk is those intent on harming themselves will simply use another method.  This raises the question would the money be better spent on improving access to mental health than spending tens of millions  on barricades?   Another interesting area to review would be are overall per capita suicide rates.  Are they higher in cities with subways than those without?  Is the main objective to save lives or limit the disruption to the train system?   The whole matter deserves a lot more thought.