Self-described "transit enthusiast" DJ Hammers was standing on the subway platform at Lexington Avenue-63rd Street on Sunday when he spotted a transit unicorn: actual, non-apparitional test cars riding along the tracks of the yes-it-will-open-in-December-maybe-we-hope-so Second Avenue Subway.

"I came across some test trains passing Lexington Avenue-63rd St to test the third rail, signals, and track on the newly-built Second Avenue Subway!" Hammers wrote in describing the video he captured.

Before we go any further, it's important to clarify that an operational track does not a complete Second Avenue Subway make. But definitely watch the video below before we finish deflating this hopeful balloon. (Fun fact: the large boxes visible on each car are weights meant to simulate the weight of passengers.)

The MTA press office didn't immediately respond to our request for comment on the test, so we called up Bill Henderson, director of the watchdog group Permanent Citizens Advisory Committee to the MTA, who was able to walk us through a typical subway completion timeline.

"My understanding is that the third rail is operational," he said, adding that rails are seldom the finishing touch:

The MTA refers "to a set of actions as the critical path for the project, which are the [actions] that determine when it gets completed. Those actions are more like putting in elevators and escalators and light safety and electronic systems, which will determine whether the line opens on time or not. Things like fire alarms and ventilating fans need to work so that if you need to exhaust smoke from the tunnel, you can do that. The fact that the MTA is testing is positive, but that doesn't mean that they've got it in the bag.

Back in July, an independent engineer testified at an MTA board meeting that there was a good chance the Second Avenue line wouldn't open in December without a revised schedule that sped up a series of unfinished system tests. Of a total 608 system tests scheduled through the end of June, only 336, or 55%, were completed by the end of that month. Then, in September, NBC reported that the MTA was adjusting expectations since elevators and escalators at the 72nd Street station won't be tested until late November. Elevators and escalators at 86th Street are also reportedly behind schedule.

"I'll let what MTA and the transit authority has said on the opening date stand for itself," Henderson said. "Officially it's still December of this year, but obviously the level of confidence is less than it was."

Henderson cited the 7 line extension as an example. "If you remember, Mayor Bloomberg, right before he went out of office, took a ride on the 7 train extension. And there were a number of months still before it opened."

The first phase of the Second Avenue Subway will run from Lexington-Avenue 63rd Street to 96th Street, primarily under Second Avenue. A second phase will eventually run from 96th Street to 125th Street.

[UPDATE 12:15 p.m.]: "We are beginning various tests all along Second Avenue Subway as we said we would," an MTA spokesperson said via e-mail. "That testing includes some train runs, which began this weekend. No change to the Dec. 31 anticipated opening date."