A device left on a London underground train was designed to cause harm, a court has heard.
Explosives expert Lorna Philp told the Old Bailey the device was intended to explode and project metal fragments into those nearby.
Student Damon Smith, 20, denies making or possessing an explosive substance with intent to endanger life.
He is accused of leaving a homemade improvised explosive device (IED) on a Jubilee Line train in October.
The jury heard the defendant told police the device was only supposed to produce smoke and said he left it on the train as a prank.
Pictures of the device were shown to jurors who were told it was made using materials including sparklers, a fairy light and a wall clock.
Ms Philp said that - in her opinion - it was "an improvised explosive device designed to explode and produce fragmentation that could cause injuries to persons and damage to property within close proximity".
It was not intended to produce smoke, she told jurors.
The presence of ball bearings indicated that "an attempt has been made" to increase the damage caused by the IED by producing "additional fragmentation to cause injuries to persons nearby," Ms Philp said.
She said an ignition section of the device was "viable" and believed it "had functioned" due to charring residue, but had failed to ignite the attached explosive substance.
Ms Philp said the device itself "could have functioned" had some elements of the design been different.
On Wednesday, the Old Bailey heard Mr Smith has Asperger's syndrome, or Autism Spectrum Disorder, and has a keen interest in guns and other weapons that may have been a function of the condition.
The trial continues.