"We basically nuked a town with chemicals so we could get a railroad open"

[ProfessorGAC]

21. There Are Flatbed Scrubbers

Some large chemical plants have them as emergency equipment so they can move it to where needed in the event they're needed.
For small venting (mild overpressure) applications, sites will use catalytic combustors. In a case like vinyl chloride, there would be a very small scrubber to turn the liberated chlorine into salt. (Usually potassium chloride, sometimes sodium, just like table salt.)
But, in a case where substantial amounts need to be mitigated, they roll up a couple of flatbeds with the hardware.
In this case, emergency teams would have the connection pieces to hook to the railcar valve at the bottom, the vent valve (has to be opened to unload the car to prevent implosion), or the ePRV (emergency pressure release valve).
They'd decide which to use by the orientation of the car. Is it on its side, leaning toward the top? Leaning toward the bottom?
I've never seen a portable combustor so I'd think we need to scrub the chemical itself not just the combustion off-gas.
Maybe(!) they could quick fabricate a flare to burn it, then run the off gas to a portable scrubber to mitigate the chlorine.
There are loads of smart people who do thus for a living & train regularly doing drills & simulations. There has to be a safe way to do this.