As of 2:32 a.m. CNN: Five devices were found near a bridge on the on the Northeast Corridor line in Elizabeth, New Jersey.  One of the robots sent to disarm a device cut a wire, and it exploded. These elaborations are given in the article:

• The devices were not equipped with activation circuitry, such as cellphones or timers, so they were incomplete.
• They were found in an area away from pedestrian traffic.
• It is surmised they were discarded.

Logical thinking alone is not enough to milk this event set. This is an example of how an open sourcer benefits greatly from an insatiable curiosity, resulting in a large general fund of knowledge. Philip Tetlock’s online crowdsourcing prediction project, Good Judgment, creates the fund by forming teams. So get together with friends.

With that knowledge, which in this case regards the characteristics of explosives, this is what comes out:

• The devices were discarded because they could not be used anymore.
• They could not be stored for future use because the explosive substance is unstable. Hence the explosion when the wire was cut.
• One substance notoriously fits: TATP, either as the main charge, or in a fuse, for which it is particularly suited . It is the favorite explosive of terrorists because it can be made out of household materials, and because it contains no telltale nitrogen.

EDIT, 8/20/2016, 1 a.m.: The initiator (fuse) has been identified (NY Times) as HMTD, another peroxide based explosive . It is more stable than TATP, except when exposed to common metals, such as the casing of a pipe bomb. This could have caused the explosion in Elizabeth when the robot cut the wire.

Unlike the use of stolen commercial explosives, TATP takes suicidal commitment at the outset. It is very dangerous to make.

It is also congruent with the NYPD controlled detonation Sunday at Rodman’s Neck. A commercial explosive would have been retained for analysis, tracing manufacture and sale. It suggests that while the bomb mechanism was sent to F.B.I. Quantico, the explosives were not, because they were too unstable to ship, and because TATP has no signature.

Practically speaking, all open source intelligence involves likelyhoods, and, at best almost-facts. There is a late, off the record statement of Tannerite residue in the 23rd Street explosion, and another citing black powder in the Seaside Park devices. Nothing has been said about the 27th Street device, where the explosive material was recovered intact. It’s up to you to decide where to place the above in this limited spectrum. Then add it to your derivation per New York Bombs, Terrorism?