According to the news reports, a plot to release cyanide gas within the New York subway system was the object of an Al Qaeda plot, which was, for reasons unknown, abandoned not long before it was to be put into operation. The state of New York, its governing body and population are now seeking a method of circumventing/dealing with such a now-possible event in the future.
I do believe the answer is already available. On an airplane, when the air within the plane becomes compromised, a sensor immediately reacts throughout the plane and oxygen masks located above each seat are released.
If this system was installed inside the entire train and released throughout the entire train at the first sensor reading of poison, the public would know when the oxygen masks were released that they should immediately be put on, with an announcement coming over the train's intercom system to follow. In the event that such a release should occur in any one car (as its source of origin), most or all of the passengers and train personnel would be safe.
The Homeland Security budget each state receives, along with tax breaks, could easily cover the retrofitting of the trains involved. This sensor system would also be executed during crashes/derailments, which would result in smoke-laden fires.
Now, while this idea is affordable, doable, simple and expedient, the existing political machinery and the penny-pinching policies of the locomotive corporate bureaucracy will be the only real impediment.