Your town no doubt has emergency evacuation plans. If they haven't shared those with you citizens, it might be time to ask them about it.

I forgot the actual numbers, but IIRC, both the accident and spill rate was considerably lower for trains than for trucks.

As far as chemicals go, I would give some serious considerations to replacing or augmenting the N95 particulate masks in the kit with better respirators (carbon filter element?). The last thing you want is to be sucking on leaking nasties and combustion products while stuck in traffic.

You can purchase an "Emergency Response Guide" which is a small book we keep in all our fire vehicles.

It gives you information about what the placards and their numbers mean, minimum safe isolation and evacuation distances, hazards and special precautions to consider and what sort of protective equipment is necessary.

A military gas mask will protect you against chemicals, biological agents, and nuclear radiated particles. Make the filters are stored in sealed plastic bags.

MSA certified full faced respirators will protect you also but make sure you have the proper filters.

Is a military gas mask required or would a commercially available full face respirator such as those available on Amazon.com be adequate. The mask I have is full face and can support particulate and various chemical filter cartridges. Filter cartridge selection really depends on what chemicals you anticipate needing to walk/drive through.

One cartridge will protect against: Organic vapors.
A second cartridge protects against Chlorine, Hydrogen chloride, Sulfur dioxide and Formaldehyde.
Another cartridge protects against: Organic vapors, Sulfur dioxide, Chlorine, Hydrogen chloride, Hydrogen fluoride.
A fourth cartridge protects against Ammonia and Methylamine vapor.

Which of those cartridges would be best to stock for a derailment event? Which of those chemicals are you most likely to encounter?

Since the cartridges are stackable you can combine the various filters to accommodate all of the above although that would be a fairly long stack and inhaling resistance would increase. I recommend adding a particulate filter and pre-filter to stop larger airborne particles from clogging the chemical filters.

continuing... the train wreck in BPJ's post had cars with Chlorine, Propane, Caustic Soda (aka Sodium hydroxide (NaOH)), and something called tooley/tooli/tooly/touli (I have no idea how to spell it and have no idea what chemicals are involved. What filter would be effective or is it best to go with an air tank and just skip filtering ambient air?

Depends... Sometimes it's know which chemicals are shipped on a railroad, so sometimes it's worth checking that out.

A respirator is a last resort kind of protection. When a gas leak occurs, make sure you are above wind and evacuate above wind. (do note, that gas leaking under pressure can travel against the wind for a certain distance)

One thing about train derailments (or big fires in industrial areas) is knowing which way the prevailing wind blows. Some of these train chemical leaks and fires unleash a deadly toxic stew that kill within seconds or minutes if breathed in.

If at all possible, you don't want to be downwind of the derailment or fire and you certainly don't want to head in that direction if under an evac.

Aside from the masks that others have mentioned, a typical urban centric BOB should suffice and will allow you to live out of house at a hotel or family/friends home for the duration of the evac which could vary from hours to weeks.

Typically these urban BOB kits should contain personal hygiene products, essential meds, OTC meds, a couple of changes of clothes which includes work clothes if required.

In addition to our BOB kits, we have a medium sized, dedicated bathroom bag that contains new toothbrushes, toothpaste, general OTC meds, deodorant, shampoo, razors, makeup (not for me!) TP, towels, washcloths and so on. The reason for this bag is so that if we ever have to do a hasty evac to a family or friends home for any duration, we won't have to go out and purchase many of these items or be a burden on our hosts to supply us with.

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.

A full face respirator with a "gets most everything" cartridge and filter combination could be useful. But it could also put you in danger of false confidence, and so increase your risk.

It's unlikely you will know exactly what you are dealing with. That's why (to my mind) hardening a residence for effective sheltering in place is really important in any such preparations.