The best test unit we have, though, will be Captain Powell.

The test unit 999 900 had been on the branch line between 2005-2009.

The testing includes verifying the physical and the logical behavior of the test unit.

The concept of this process was initiated in 1986 in the test unit.

Unit tests are so named because they each test one unit of code.

They were used as test units between their delivery and 1995, when the first production sets entered into service.

But the test unit's print quality was disappointing by today's standards.

"And we're sending 30 test units this month to London."

Four test units were excavated in a period of two weeks.

It was pulled from service in 2008 for undisclosed reasons, but may have simply been a test unit on loan.