The sign reads "Train on trial run - do not board." A look at the Wikipedia entry for Line 9 revealed this welcome bit of information:
Currently [sic] there are 36 trains with four cars each. The express train comes every 20 minutes and the all-stop train comes every 6.7 minutes. Due to the number of riders exceeding 250,000 per day, 12 additional four car trains have been ordered and are expected to be in service starting October, 2011. This would mean the express train comes every 7~10 minutes and the all-stop train comes every 5 minutes.This Munhwa Ilbo article is listed as a source, which makes clear that, when the 12 new trains are put into service in October, there will be 36 trains in total, meaning there must be 24 trains at the moment (most likely 8 express and 16 all-stop trains). It also notes that the new trains would arrive in July 2011, 16 months after the order was placed for them (ie March 2010). I imagine they've arrived early, and that the train above is one of them.
The increase in the number of trains is welcome news. The express trains are jam-packed to the point they can no longer take on passengers (after two stops!) in the mornings, and are little better in the evenings. One reason for this is that the trains are only four cars long, but the stations have room for trains with eight cars. I'd wondered if this was a bid by the company to save money, seeing as it initially planned, if I remember correctly, to charge a fare of 1,300 won, but was forced by the city to charge 900 won (it's vaguely mentioned here). Perhaps this isn't the case, however. I think increasing the number of trains, rather than the number of cars, is the best way to go. The trains come infrequently compared to other subway lines (all-stop trains leave Gimpo Airport Station every 10 minutes, with express trains leaving every 20 minutes). Needless to say, I'm looking forward to the change, though as far as going downtown is concerned, the AREX line will always be faster, especially when the transfer at Gongdeok station opens at the end of this year. The two lines together have made the Gangseo-gu area feel much closer to downtown, and have affected housing prices as well.
One nice thing about the Line 9 trains is being able to look out the front window...