Some of these hazardous materials displace oxygen -- a filter on a gas mask (military or otherwise) won't help when there's no oxygen in the environment to breathe. For a variety of scenarios (depending on the HAZMAT), you'd have to bring your own air -- a self-contained breathing apparatus (SCBA) like firefighters and HAZMAT techs wear would be indicated.

SCBA will protect you against a wider gamut of HAZMAT threats than a gas mask, but the downside is that you need some infrastructure to change and refill bottles -- and with both methods, how do you change the filter or bottle without a safe zone or decontamination facilities? For all practical purposes, you'd have to be out of the hot zone before it becomes necessary, and since you can burn through a 30 minute SCBA bottle (just for an example) in 15 minutes depending on how much physical activity you're doing, it doesn't buy you a whole lot of time.

So you can certainly buy a gas mask, or SCBA, for your own personal use. You can even find them on Ebay.

But I guess what I'm having trouble picturing is what sort of scenario do you envision where one of these would save you, where otherwise you would have perished?

Some considerations:

1. Will it be immediately accessible at all times? (ie, while you're grocery shopping?)

2. How will you be reliably warned that you'd need to don the gear? If the train derails right next to your house, grabbing your go-bag, getting in the car and leaving immediately would likely allow you to escape the immediate danger even without a gas mask or SCBA. We train to put our bunker gear and SCBA on in less than 2 minutes, and that's sitting there in front of me -- but I could also be 2 miles down the road in that amount of time (especially if other people aren't as quick to react and never prepared or don't know what to do -- so traffic is probably not such a major issue yet)

3. If it derails further up or down the rail and you can hear or see indications that it has derailed in the distance (like a cloud or smoke rising), the same applies. If it's too far to hear and you're downwind and out shopping and your only indication is people start falling out around you -- it doesn't do much good then either.

4. Will your loved ones have gear too?

It would take an awfully particular set of circumstances for this gear to be useful, and it would have significant caveats and limitations to consider. If, in your mind, the odds of you needing it outweigh the likelihood that you would die without it then by all means go for it, but your time and money might be better spent in other areas. Having enough money on hand for you and your family to stay in a hotel for a week during an evacuation would put you a step ahead of most people.