It is completely impossible to create, modify or otherwise manufacture a disease which can cause death within two minutes of exposure or infection. Not even the most virulent and deadliest of viruses can act that fast. This isn't pseudo-science as stated in the show - it's sheer fantasy.
If you don't want to be nauseated in a morgue, you should eat. Empty stomach increases nausea.
The Washington DC Metro is shown with seating along the walls, facing inward. The Metro has seats arranged in rows facing the front and back of the car, like an intercity train.
Strontium-90 could not even significantly affect the human immune system within the space of a minute or two if there was enough of it on the subway train to give everybody acute radiation poisoning, let alone "trace amounts". The reason it affects the immune system at all is because it is incorporated into bone and bone marrow because it is chemically similar to calcium. Which obviously takes weeks and months.
Black phosphorus is not some sort of bio-chemical agent but a stable modification of a very common element that was first synthesized in 1914.
The subway that was the focus of this episode is not the Washington DC Metrorail but the New York City Subway. Both systems have very distinctive physical appearances. The DC Metro is much more open and brighter than the NYC transit system as it is a newer system.
At the beginning of the episode a public address speaker announces connecting trains to the orange and blue lines at Farragut East and to the yellow line at Farragut North. There is no Farragut East station and Farragut North is not a connecting station.
Most if not all courthouses have security checks for entry these days. It is VERY unlikely that Barnes would have been able to stroll into the courthouse carrying the briefcase without having to face a check that would not have allowed him to proceed without examining the briefcase.