By removing the shielding to make it dangerous to others you start
a 30 second clock on yourself.
A 30-second clock to receiving the LD50, not a 30-second clock to actual death. Radiation poisoning doesn't kill immediately, and even the symptoms (burns, etc.) don't show up that quickly. It would likely take several dozen hours before an individual actually died from such exposure, plenty of time to get off the train and go find a quick way to die.
Yes it is true you do not go poof in 30 seconds but after thirty seconds your fate would be cast.
Thirty seconds is a rather short length of time about the length of time to walk 175 feet.
I do not know how long it takes to be debilitated but in terms of getting anything done
the clock is not your friend. WP hints... "absorbed doses of 6–30 Gy (600–3000 rad).
Nausea, vomiting, loss of appetite, and abdominal pain are usually seen within two hours."
From the images the mass of the container is substantial and beyond the capacity of
a common auto.
I think this is a very nasty business and do not think it is productive to think too hard
about how to do it on a public forum.
A source like this is easy and inexpensive to detect short of it being encased in serious
mass perhaps a ton of steel concrete or lead. That in turn is not stealthy or easy to hide...
Alpha emitters on the other hand are quite easy to shield by way of comparison.
Those charged with public safety cannot ignore this but we seem to tolerate 38,000 deaths
with automobiles so this risk almost seems manageable.