oh yes yes ricin is soooo credible in a terror attack
is that the fact that at 3% medium concentration (10% for specificly engineered species) of ricin in ricinus seeds makes pretty spottable the refining ("good morning mister, i would like three tons of castor beans.") of the substance ? no no, let's assume they dispose already of the refined form.
so with a ton of ricinus they will have 30Kg equivalent of somewhat pure ricin. ok now the hard part
LD50 is 1mg/Kg of matter by inhalation or injection means (500mg for ingestion). unlike the Markov case, where the poison was directly administered by a small hollow pellet *directly into the circulatory system, the only somewhat realistic way you'll have to conduct a real terror attack is by aerosol.
so assuming a crowded train station (with your average terrorist being ready do die in horrible suffering by the action of his bomb) you'll have to *diffuse* with appropriate means (compressed gaz with double action valves (we wouldn't want to kill our own jihadi in the lab wouldn't we ?) or low explosive powered deflagration (black powder, nitrate/fuel etc...) with 160Kg of body matter per cubic meter at ground (probably way more at busy hours)
so assuming you want to fill 100 cubic meters of your aerosol that would represent a whooping 160 Kg of ricin to aerosolize by appropriate means (count double weight around effective payload for appropriate aerosolizer/propellant/container)
OH MY GOD NO, HULK HAS A BEARD AND A GINORMOUS SPRAY CAN OF FLY TOX
of course you can consider my ramblings as totally irrelevant. or not.
oh btw a nitrate/fuel bomb is far easier to make, and pretty effective blowing people apart. ask the IRA